Modern, distributed systems are complex and present numerous challenges: concurrency, process crashes, message loss, duplication, or reordering. How can developers confidently test distributed systems instead of continuously dreading the next hard-to-catch and hard-to-reproduce Heisenbug? Deterministic Simulation Testing is a powerful testing technique that eliminates uncertainty－or rather non-determinism－and ensures that your system is tested exhaustively and every single test is reproducible. Using a systems modeling approach, we will accurately and concisely discuss Deterministic Simulation Testing. In addition, we will explore real-world implementation of this technique in production in projects such as FoundationDB, TigerBeetle, and Resonate. Gain actionable insights for crafting your own Deterministic Simulation Testing strategy. Confidently open Schrödinger's Box of testing in distributed systems.