From list sorting to network routing, and from hash tables to capacity planning, a programmer's daily work is filled with probability. We use probabilistic algorithms, data structures, and systems constantly often without even thinking about it. Experienced engineers reach for probabilistic algorithms frequently and intentionally, especially when building systems of serious scale. How do probabilistic algorithms actually work in practice? And how do we know they'll be safe and reliable in our critical production systems? We'll address those questions, explore a few algorithms, and see why "with high probability" is often better than "exactly".