common point of confusion among Ruby developers; namely: Concurrency and parallelism are not the same thing (i.e., concurrent != parallel). In particular, Ruby concurrency is when two tasks can start, run, and complete in overlapping time periods. It doesn’t necessarily mean, though, that they’ll ever both be running at the same instant (e.g., multiple threads on a single-core machine). In contrast, parallelism is when two tasks literally run at the same time (e.g., multiple threads on a multicore processor). The key point here is that concurrent threads and/or processes will not necessarily be running in parallel. This tutorial provides a practical (rather than theoretical) treatment of the various techniques and approaches that are available for concurrency and parallelism in Ruby.