Inclusion Criteria:

          -  Participant must have an advanced solid tumor and must not be a candidate for surgical
             resection or other approved therapeutic regimen known to provide clinical benefit. For
             dose escalation, the participant may have been previously treated with a programmed
             cell death 1 (PD-I) targeting agent. For dose expansion, the participant must be
             PD-I/PD-L1 targeting agent naïve. For Part 2 ABBV-181 in combination with
             rovalpituzumab tesirine, the participant must have SCLC with progressive disease and
             have failed platinum containing therapy and be PD-1/PD-L1 targeting agent naïve. For
             Part 3 ABBV-181 in combination with venetoclax, the participant must have locally
             advanced or metastatic NSCLC and received no more than four lines of therapy in an
             advanced or metastatic setting and has progressed on no more than one PD-1/PD-L1
             containing therapy.

          -  Participant has an Eastern Cooperative Oncology Group (ECOG) Performance Status of 0
             to 2 for the monotherapy cohort and an ECOG 0 to 1 for ABBV-181 in combination with
             rovalpituzumab tesirine cohort (Part 2) and ABBV-181 in combination with venetoclax
             (Part 3).

          -  Participants have adequate bone marrow, renal, hepatic and coagulation function.

          -  Participants must have measurable or evaluable disease per Response Evaluation
             Criteria in Solid Tumors (RECIST) version 1.1 in the dose escalation portion of the
             trial. Participants in the expansion cohort must have measurable disease per RECIST
             version 1.1 or disease evaluable by assessment of tumor antigens. Participants
             enrolled in ABBV-181 in combination with venetoclax cohort (Part 3) must have
             measurable disease per RECIST version 1.1.

        Exclusion Criteria:

          -  Participant has received anticancer therapy including chemotherapy, immunotherapy,
             radiation therapy, biologic, herbal therapy, or any investigational therapy within a
             period of 5 half-lives, prior to the first dose of ABBV-181 or Rovalpituzumab Tesirine
             or venetoclax.

          -  For ABBV-181 plus rovalpituzumab tesirine therapy (Part 2), participant must not have
             had prior exposure to Rovalpituzumab Tesirine or a pyrrolobenzodiazepine (PBD) based
             drug.

          -  Participant has unresolved adverse events greater than grade 1 from prior anticancer
             therapy except for alopecia.

          -  Current or prior use of immunosuppressive medication within 14 days prior to the first
             dose (with certain exceptions).

          -  History of primary immunodeficiency, bone marrow transplantation, chronic lymphocytic
             leukemia, solid organ transplantation, or previous clinical diagnosis of tuberculosis.

          -  Confirmed positive test results for human immunodeficiency virus (HIV), or
             participants with chronic or active hepatitis A, B or C. Subjects who have a history
             of hepatitis B or C who have undetectable HBV DNA or HCV RNA are anti-viral therapy
             may be enrolled.

          -  Participant has known history or inflammatory bowel disease, pneumonitis, or known
             uncontrolled metastases to the central nervous system (CNS) (with certain exceptions).

          -  Participants with a history of or ongoing pneumonitis or interstitial lung disease are
             also excluded.

          -  For ABBV-181 plus venetoclax therapy (Part 3), participant must not receive a strong
             or moderate inducer or inhibitor of cytochrome P450 (CYP)3A within 7 days before first
             venetoclax dose.

          -  For ABBV-181 plus venetoclax therapy (Part 3), participants with a known
             gastrointestinal disorder (i.e.: malabsorption syndrome), complication (i.e.:
             dysphagia) or surgery that could make consumption or absorption of oral medication
             problematic are also excluded.