This book constitutes the refereed proceedings of the12th International Colloquium on Theoretical Aspects of Computing, ICTAC 2015,held in Cali, Colombia, in October 2015.The 25 revised full papers presented togetherwith 7 invited talks, 3 tool papers, and 2 short papers were carefully reviewedand selected from 93 submissions. The papers cover various topics such asalgebra and category theory; automata and formal languages; concurrency;constraints, logic and semantic; software architecture and component-baseddesign; and verification.