Inclusion Criteria:

          -  Age ≥ 18 years at time of informed consent

          -  Body weight > 30 kg

          -  Histologically or cytologically confirmed small cell carcinoma, squamous cell
             carcinoma or adenocarcinoma (confirmed at MSKCC) of the bladder, ureter, urethra,
             urachus, or renal pelvis. Patients with squamous cell carcinoma and adenocarcinoma are
             required to have a predominant squamous or adenocarcinoma component as reviewed by the
             pathologist at MSKCC. However, if any element of small cell or neuroendocrine
             differentiation is present, the patients will be classified as small
             cell/neuroendocrine.

          -  Confirmation of availability of sufficient tissue from a prior surgery for correlative
             studies is required prior to enrollment. Patients must have representative non-TCC or
             the urothelial tract FFPE archival tumor specimens (tumor blocks or 30 unstained
             slides; preference for tumor blocks). These samples may be submitted between the time
             of consent and the start of treatment. Patients with < 30 slides may be enrolled after
             discussion with the principal or co-principal investigators.

          -  Clinical evidence of metastatic (T4b, any N; any T, N2-3; M1) disease.

          -  Life expectancy of 12 weeks of greater based on assessment by the treating
             investigator.

          -  Evidence of measurable disease by RECIST 1.1.

          -  Patients with small cell carcinoma must have progressed after at least one prior
             systemic therapy. Patients with squamous cell carcinoma or adenocarcinoma may be
             previously untreated or have progressed after prior systemic therapy. Chemotherapy
             administered in conjunction with primary radiation as a radio-sensitizer WILL be
             counted as a systemic chemotherapy regimen. NOTE: There is no maximum number of prior
             treatments allowed.

          -  Patients with brain metastases are allowed onto the study as long as patients have
             completed their treatment for brain metastasis, no longer require corticosteroids, and
             are asymptomatic. Subjects with neurological symptoms should undergo a head CT scan or
             brain MRI to exclude brain metastasis, at the discretion of the treating physician.

          -  Eastern Cooperative Oncology Group (ECOG) performance status of 0-1.

          -  Adequate normal organ and marrow function as defined below:

               -  Hemoglobin ≥ 9.0 g/dL

               -  Absolute neutrophil count (ANC) ≥ 1.0 x 109/L (> 1000 per mm^3)

               -  Platelet count ≥ 100 x 109/L (>100,000 per mm^3)

               -  Serum bilirubin ≤ 1.5 x institutional upper limit of normal (ULN) (≤ 3 x
                  institutional ULN in patients with Gilbert"s syndrome)

               -  AST (SGOT)/ALT (SGPT) ≤ 2.5 x institutional ULN unless liver metastases are
                  present, in which case it must be ≤ 5 x ULN

               -  Calculated creatinine clearance > 30 mL/min by the Cockcroft-Gault formula
                  (Cockcroft and Gault 1976) or by 24-hour urine collection for determination of
                  creatinine clearance:

        Males:

        Creatinine CL (mL/min)

        = Weight (kg) x (140 - Age) . 72 x serum creatinine (mg/dL)

        Females:

        Creatinine CL (mL/min)

          -  Weight (kg) x (140 - Age) x 0.85 72 x serum creatinine (mg/dL)

               -  Evidence of post-menopausal status or negative serum pregnancy test for female
                  pre-menopausal patients. Women will be considered post-menopausal if they have
                  been amenorrheic for 12 months without an alternative medical cause. The
                  following age-specific requirements apply:

                    -  Women <50 years of age would be considered post-menopausal if they have been
                       amenorrheic for 12 months or more following cessation of exogenous hormonal
                       treatments and if they have luteinizing hormone and follicle-stimulating
                       hormone levels in the post-menopausal range for the institution or underwent
                       surgical sterilization (bilateral oophorectomy or hysterectomy).

                    -  Women ≥50 years of age would be considered post-menopausal if they have been
                       amenorrheic for 12 months or more following cessation of all exogenous
                       hormonal treatments, had radiation-induced menopause with last menses >1
                       year ago, had chemotherapy-induced menopause with last menses >1 year ago,
                       or underwent surgical sterilization (bilateral oophorectomy, bilateral
                       salpingectomy or hysterectomy).

               -  Female patients of reproductive potential and non-sterilized males who are
                  sexually active with a female partner of childbearing potential must be willing
                  to adhere to the following restrictions:

                    -  Females of reproductive potential who are sexually active with a
                       non-sterilized male partner must agree to use at least 1 highly effective
                       method of contraception (Table 3) from the time of screening until 180 days
                       after the last dose of durvalumab + tremelimumab combination therapy or 90
                       days after the last dose of durvalumab monotherapy. Cessation of birth
                       control after this point should be discussed with a responsible physician.
                       Periodic abstinence, the rhythm method, and the withdrawal method are not
                       acceptable methods of birth control. It is strongly recommended that
                       non-sterilized male partners of a female patient must use male condom plus
                       spermicide throughout this period. Not engaging in sexual activity for the
                       total duration of the drug treatment and the drug washout period is an
                       acceptable practice.

                    -  Non-sterilized males who are sexually active with a female partner of
                       childbearing potential must use a male condom plus spermicide from screening
                       through 180 days after receipt of the final dose of durvalumab +
                       tremelimumab combination therapy or 90 days after receipt of the final dose
                       of durvalumab monotherapy. Periodic abstinence, the rhythm method, and the
                       withdrawal method are not acceptable methods of birth control. Not engaging
                       in sexual activity is an acceptable practice. Male patients should refrain
                       from sperm donation throughout this period. It is strongly recommended that
                       female partners (of childbearing potential) of male patients to also use a
                       highly effective method of contraception throughout this period.

                    -  Highly effective methods of contraception, defined as one that results in a
                       low failure rate (i.e., less than 1% per year) when used consistently and
                       correctly.

               -  Subject is willing and able to comply with the protocol for the duration of the
                  study including undergoing treatment and scheduled visits and examinations
                  including follow up.

        Exclusion Criteria:

          -  Previous enrollment in the present study

          -  Participation in another clinical study with an investigational product during the
             last 14 days

          -  Mean QT interval corrected for heart rate (QTc) ≥470 ms calculated from 3
             electrocardiograms (ECGs) using Fredericia"s Correction

          -  Any previous treatment with a PD-1 or PD-L1 inhibitor, including durvalumab or an
             anti-CTLA-4, including tremelimumab

          -  Receipt of the last dose of anti-cancer therapy (chemotherapy, immunotherapy,
             endocrine therapy, targeted therapy, biologic therapy, tumor embolization, monoclonal
             antibodies, other investigational agent) < 21 days prior to enrollment.

          -  Major surgery within 28 days of starting study treatment. There is no minimum time
             requirement for minor procedures such as biopsy or vascular access placement.

          -  Radiation within 14 days of starting study treatment

          -  Current or prior use of immunosuppressive medication within 14 days before the first
             dose of durvalumab or tremelimumab. The following are exceptions to this criterion:

               -  Intranasal, inhaled, topical steroids, or local steroid injections (e.g., intra
                  articular injection)

               -  Systemic corticosteroids at physiologic doses not to exceed <<10 mg/day>> of
                  prednisone or its equivalent

               -  Steroids as premedication for hypersensitivity reactions (e.g., CT scan
                  premedication)

          -  Any unresolved toxicity from previous anti-cancer therapy must have resolved to at
             least ≤ Grade 1 (or baseline) at time of enrollment.

        Patients with irreversible toxicity that is not reasonably expected to be exacerbated by
        treatment with durvalumab and tremelimumab may be included after consultation with the
        Principal Investigator or Co-Principal Investigator (e.g. alopecia, hearing loss,
        peripheral neuropathy).

          -  Active or prior documented autoimmune or inflammatory disorders (including
             inflammatory bowel disease [e.g., colitis or Crohn's disease], diverticulitis, celiac
             disease, systemic lupus erythematosus, Sarcoidosis syndrome, or Wegener syndrome
             [granulomatosis with polyangiitis, Graves' disease, rheumatoid arthritis,
             hypophysitis, uveitis, etc]). The following are exceptions to this criterion:

               -  Patients with vitiligo or alopecia

               -  Patients with hypothyroidism (eg, following Hashimoto syndrome) stable on hormone
                  replacement

               -  Any chronic skin condition that does not require systemic therapy

               -  Patients without active autoimmune disease in the last 5 years may be included
                  but only after consultation with the study physician

               -  Patients with diverticulosis

               -  Patients with celiac disease controlled by diet alone

          -  History of primary immunodeficiency

          -  History of allogeneic organ transplant

          -  History of hypersensitivity to durvalumab, tremelimumab or any excipient

          -  Uncontrolled intercurrent illness, including but not limited to, ongoing or active
             infection, symptomatic congestive heart failure, uncontrolled hypertension, unstable
             angina pectoris, cardiac arrhythmia, interstitial lung disease, serious chronic
             gastrointestinal conditions associated with diarrhea, or psychiatric illness/social
             situations that would limit compliance with study requirement, substantially increase
             risk of incurring AEs or compromise the ability of the patient to give written
             informed consent.

          -  Active infection including tuberculosis (clinical evaluation that includes clinical
             history, physical examination and radiographic findings, and TB testing in line with
             local practice), hepatitis B (known positive HBV surface antigen (HBsAg) result),
             hepatitis C, or human immunodeficiency virus (positive HIV 1/2 antibodies). Patients
             with a past or resolved HBV infection (defined as the presence of hepatitis B core
             antibody [anti-HBc] and absence of HBsAg) are eligible. Patients positive for
             hepatitis C (HCV) antibody are eligible only if polymerase chain reaction is negative
             for HCV RNA.

          -  History of leptomeningeal carcinomatosis

          -  Receipt of live attenuated vaccination within 30 days prior to study entry or within
             30 days of receiving durvalumab or tremelimumab. Inactivated vaccines, such as the
             injectable influenza vaccine, are permitted.

          -  Any condition that, in the opinion of the investigator, would interfere with
             evaluation of study treatment or interpretation of patient safety or study results

          -  Malignancies other than the disease under study within 5 years prior to Cycle 1, Day
             1, with the exception of those with a negligible risk of metastasis or death and with
             expected curative outcome (such as adequately treated carcinoma in situ of the cervix,
             basal or squamous cell skin cancer, localized prostate cancer treated surgically with
             curative intent, or ductal carcinoma in situ treated surgically with curative intent)
             or undergoing active surveillance per standard-of-care management (e.g. prostate
             cancer with Gleason score ≤ 6, and prostate-specific antigen [PSA] ≤ 10 mg/mL, etc).

          -  Patients should agree to not donate blood while participating in this study or for at
             least 90 days following the last infusion of durvalumab or tremelimumab

          -  Female patients who are pregnant or breastfeeding or male or female patients of
             reproductive potential who are not willing to employ effective birth control from
             screening to 90 days after the last dose of durvalumab monotherapy or180 days after
             the last dose of durvalumab + tremelimumab combination therapy.