This is a Phase 1/2, multicenter, open-label study to evaluate the safety and efficacy of BEAM-201 in patients with relapsed/refractory T-ALL or T-LL. This study consists of Phase 1 dose-exploration cohorts, Phase 1 dose-expansion cohort(s), a Phase 1 pediatric cohort (will enroll patients ages 1 to \< 12 years), and a Phase 2 cohort.