Wwrts Introduction → You're Always in a Thread → Threads of Execution → Lifecycle of a Thread → Concurrent != Parallel → The GIL and MRI → How Many Threads Are Too Many? → Real Parallel Threading With JRuby and Rubinius → Thread Safety → Protecting Data With Mutexes → Signaling Threads With Condition Variables → Thread-Safe Data Structures → Writing Thread-Safe Code → Thread-Safety on Rails → Wrap Your Threads in an Abstraction → How Sidekiq Uses Celluloid → Puma's Thread Pool Implementation → Closing → Appendix: Atomic Compare-and-Set Operations → Appendix: Thread-Safety and Immutability →