This volume constitutes the thoroughly refereed proceedings of the 21st International Workshop on Cellular Automata and Discrete Complex Systems, AUTOMATA 2015, held in Turku, Finland, in June 2015. This volume contains 4 invited talks in full-paper length and 15 regular papers, which were carefully reviewed and selected from a total of 33 submissions. Topics of interest include, the following aspects and features of such systems: dynamical, topological, ergodic and algebraic aspects
algorithmic and complexity issues
emergent properties
formal language processing aspects
symbolic dynamics
models of parallelism and distributed systems
timing schemes
phenomenological descriptions
scientific modeling
and practical applications.