Chimeric antigen receptor (CAR) T cell therapy is a novel therapy for patients with relapsed or refractory hematologic malignancies. Most CAR T cell therapy recipients will experience clinical features of the immune effector cell-associated neurotoxicity syndrome (ICANS), a potentially life-threatening condition. Here we describe the clinical, biological, and radiological findings associated with ICANS in adults with hematologic malignancies treated with CAR T cell therapy, as well as the acute and long-term outcomes of ICANS. A literature search of Ovid Medline, Embase, PubMed, Scopus, Web of Science Core Collection, Cochrane Library, and Google Scholar was conducted from each database's inception through February 1, 2022, using search terms reflecting CAR T cell therapy and ICANS. We included studies that enrolled adults (age ≥18 years) who received CAR T cell therapy as management for hematologic malignancies and reported the clinical presentation, predictors, and/or acute or long-term outcomes of ICANS. Two reviewers independently extracted data following the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) reporting guidelines. Quality was assessed using the Joanna Briggs Institute critical appraisal tool for cohort studies. Of the 2928 studies screened, 23 observational studies (10 prospective, 11 retrospective, 1 mixed design, and 1 cross-sectional) with a total of 1666 participants met our eligibility criteria and were included in our review. The most common hematologic malignancies were diffuse large B cell lymphoma, acute lymphocytic leukemia, non-Hodgkin lymphoma, and chronic lymphocytic leukemia. ICANS onset was most often associated with the presence and severity of cytokine release syndrome, as well as with C-reactive protein and ferritin levels. Aphasia was the most common ICANS-related symptom reported, although the neurologic manifestations of ICANS were highly variable. Neuroimaging studies (magnetic resonance imaging or computed tomography) were often normal in cases of ICANS; however, electroencephalography often showed generalized background slowing, abnormal rhythmic, and periodic discharge patterns. The pooled mean (± SD) onset of ICANS was 6.4 ± 3.2 days, with a pooled mean duration of 8.3 ± 10.5 days. Two of the 23 studies (9%) reported 5 ICANS-related deaths among 233 participants. A subset of patients experienced persistent neurocognitive complaints at ≥1-year after CAR T cell therapy. The clinical presentation, onset, severity, long-term sequelae, and grading system of ICANS are variable. Future studies should consider using a consensus grading/reporting scale that would permit cross-trial comparisons of the safety profile of various CAR T cell products and enable the development of interventions to mitigate or manage these neurotoxicities. © 2022 American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc. This systematic review was conducted according to a published protocol (PROSPERO CRD42020207864) and followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) and Synthesis without Meta-Analysis (SWiM) in systematic review reporting guidelines (Supplementary Table S1) [15,16].