This book constitutes the refereed proceedings of the 15th International Colloquium on Theoretical Aspects of Computing, ICTAC 2018, held in Stellenbosch, South Africa, in October 2018. The 25 revised full papers presented together with two short and two long invited talks were carefully reviewed and selected from 59 submissions. The ICTAC conference aims at bringing together researchers and practitioners from academia, industry and government to present research and exchange ideas and experience addressing challenges in both theoretical aspects of computing and the exploitation of theory through methods and tools for system development. ICTAC also specifically aims to promote research cooperation between developing and industrial countries.