PaperAbstract. High-performance computing systems have historically been designed to support applications comprised of mostly monolithic, singlejob workloads. Pilot systems decouple workload specification, resource selection, and task execution via job placeholders and late-binding. Pilot systems help to satisfy the resource requirements of workloads comprised of multiple tasks. RADICAL-Pilot (RP) is a modular and extensible Python-based pilot system. In this paper we describe RP’s design, architecture and implementation, and characterize its performance. RP is capable of spawning more than 100 tasks/second and the steady-state execution of up to 16K concurrent tasks. RP can be used stand-alone, as well as integrated with other application-level tools as a runtime system.