(with Marc Shapiro) Distributed systems face a fundamental trade-off of consistency vs. availability and performance. Strong consistency is easy to understand but is slow, expensive, and is unavailable when the system partitions. Eventual consistency (EC) can be cheaper, faster, and more scalable, but is hard to understand and get correct. This tutorial explores the multiple gradations between strong and eventual consistency. It focuses on understanding EC, from perspectives of the algorithm designer, of the system builder, and the application programmer. It will include formal definitions of correctness, study of lower bounds, and implementation recipes and tricks.