This document discusses concurrency patterns and issues in multi-threaded programming. It introduces common concurrency primitives like threads, mutexes, and condition variables. It describes problems that can occur with race conditions and memory consistency. Several patterns for managing concurrent access are explained, including fully synchronized objects, resource ordering, copy-on-write, partitioning, and lock-free queues. The document cautions that any pattern requiring complex analysis to prove thread-safety may be fragile.