This book constitutes the refereed proceedings of the 20th International Conference on Implementation and Application of Automata, CIAA 2015, held in held in Umeå, Sweden, in August 2015. The 22 revised full papers presented together with 4 invited papers and 2 toool demonstration papers were carefully reviewed and selected from 49 submissions. The papers cover all aspects of cover automata, counter automata, decision algorithms on automata, descriptional complexity, expressive power of automata, homing sequences, jumping finite automata, multi-dimensional languages, parsing and pattern matching, quantum automata, realtime pushdown automata, random generation of automata, regular expressions, security issues, sensors in automata, transducers, transformation of automata, and weighted automata.