The Sprite operating system allows executing processes to be moved between hosts at any time. We use this process migration mechanism to offload work onto idle machines, and also to evict migrated processes when idle workstations are reclaimed by their owners. Sprite's migration mechanism provides a high degree of transparency both for migrated processes and for users. Idle machines are identified, and eviction is invoked, automatically by daemon processes. On Sprite it takes 40-50 milliseconds on DECstation 3100 workstations to perform a remote exec. The pmake program uses exec-time migration to invoke compilations and other tasks concurrently, commonly producing speed-up factors in the range of three to six. Process migration has been in regular service for over a year.