• Signed informed consent

• Relapsed or refractory solid tumor after at least one prior course of therapy.

∙ Hodgkin lymphoma or non-Hodgkin lymphoma are not permitted.

‣ Patients with CNS malignancy or asymptomatic CNS metastases may be enrolled, provided all of the following criteria are met.

⁃ No metastatic or primary disease affecting the brainstem, midbrain, pons, or cerebellum, or within 10 mm of optic nerve

• No history of leptomeningeal disease

• No history of intracranial or spinal cord hemorrhage

• No evidence of progression of neurologic deficit, in the investigator's judgment, within 7 days prior to initiation of study medications.

‣ Must have histologically confirmed rhabdomyosarcoma (RMS) for RMS efficacy cohort.

• Age ≥ 6 months and ≤ 30 years

• Lansky Performance Status (patients \< 16 years old) or Karnofsky Performance Status (patients ≥ 16 years old) ≥ 50

• Ability to comply with the study protocol, in the investigator's judgment

• For RMS efficacy cohort, disease must be measurable as defined by RECIST v1.1.

∙ For the feasibility cohort, disease must be evaluable, but patients enrolled in the feasibility cohort will be prospectively assessed for measurable disease, RMS patients will also be included in the RMS efficacy cohort.

‣ Previously irradiated lesions can be considered as measurable disease only if progressive disease has been unequivocally documented at that site since radiation.

• Availability of a tumor specimen suitable for determination of PD-L1 status, either from initial diagnosis or from a recurrence.

∙ For PD-L1 staining to be performed at the central site, a formalin-fixed paraffin-embedded (FFPE) tumor specimen in a paraffin block (preferred) or at least 15 slides containing unstained, freshly cut, serial sections must be available along with an associated pathology report prior to study enrollment.

‣ Patients for whom the required number of slides are not available may still be eligible to enroll on study with PI approval

• For the RMS efficacy cohort, it will be required that at least 8 of 17 patients have PD-L1(+) tumor. PD-L1 status will be determined at time of enrollment for all patients. When the maximum allowable number of PD-L1(-) patients has been enrolled and treated on study, PD-L1 positivity will be required for all further enrolled patients.

∙ Staining will be performed in the central site CAP/CLIA-certified laboratory using the 22c3 antibody for immunohistochemical analysis

‣ PD-L1(+) status will be defined as staining on ≥1% of tumor cells or ≥1% of stroma.

‣ For the feasibility cohort, PD-L1 positivity is not required but will be performed centrally in all cases for exploratory biomarker studies.

• Adequate organ and marrow function as defined by the following laboratory values obtained within 21 days prior to initiation of study medication.

∙ For patients without known bone marrow involvement:

⁃ Absolute neutrophil count ≥ 1.0 x 10\^9 / L (1000/µL) without granulocyte colony-stimulating factor support (≥14 days after the last dose of a long-acting growth factor such as pegfilgrastim, or 7 days after short-acting growth factor)

• Platelet count ≥ 75 x 10\^9 / L (75,000/µL) without transfusion in the last 7 days

‣ Patients with known bone marrow metastatic disease will be eligible for the study if they meet the following criteria:

⁃ Patients with documented liver metastases: AST and ALT ≤ 5 x ULN

• Patients with documented liver or bone metastases: ALP ≤ 5 x ULN

• Absolute neutrophil count (ANC) ≥ 750/mm\^3

• Platelet count ≥ 50,000/mm\^3 (may receive transfusions provided they are not known to be refractory to red cell or platelet transfusions)

• These patients will not be evaluable for hematologic toxicity. At least 4 of 6 patients in the feasibility cohort must be evaluable for hematologic toxicity. If dose-limiting hematologic toxicity is observed, all subsequent patients enrolled must be evaluable for hematologic toxicity.

‣ Total bilirubin ≤1.5 x upper limit of normal (ULN) for age (Patients with known Gilbert disease: serum bilirubin ≤ 3 x ULN)

‣ AST (SGOT) and ALT (SPGT) ≤ 2.5 x ULN for age

‣ Serum albumin ≥ 25 g/L (2.5 g/dL)

‣ Creatinine ≤ 1.5 x ULN for age or creatinine clearance (or radioisotope glomerular filtration rate) ≥ 70 mL/min/1.73 m2

‣ Left ventricular ejection fraction ≥ 50% or shortening fraction ≥ 30%

‣ Hemoglobin ≥ 90 g/L (9 g/dL)

‣ Patients may be transfused to meet this criterion.

∙ For patients not receiving therapeutic anticoagulation: INR or aPTT ≤ 1.5 x ULN

∙ For patients receiving therapeutic anticoagulation: stable anticoagulant regimen

⁃ Negative HIV and hepatitis B surface antigen (HBsAg) tests at screening

⁃ For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive methods, and agreement to refrain from donating eggs, as defined below:

• Women must remain abstinent or use contraceptive methods with a failure rate of \< 1% per year during the treatment period and for 5 months after the final doses of atezolizumab, vincristine, and temozolomide. Women must refrain from donating eggs during this same period.

∙ A woman is considered to be of childbearing potential if she is postmenarchal, has not reached a postmenopausal state (≥ 12 continuous months of amenorrhea with no identified cause other than menopause), and has not undergone surgical sterilization (removal of ovaries and/or uterus), regardless of sexual orientation or marital status.

∙ Examples of contraceptive methods with a failure rate of \&amp;lt; 1% per year include bilateral tubal ligation, male sterilization, hormonal contraceptives that inhibit ovulation, hormone-releasing intrauterine devices, and copper intrauterine devices.

∙ The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not adequate methods of contraception.

⁃ For men who are not surgically sterile: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures, and agreement to refrain from donating sperm, as defined below:

• With a female partner of childbearing potential who is not pregnant, men must remain abstinent or use a condom plus an additional contraceptive method that together result in a failure rate of less 1% per year during the treatment period and for 5 months after the final doses of atezolizumab, irinotecan, and temozolomide. Men must refrain from donating sperm during this same period.

∙ The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not adequate methods of contraception