This book constitutes the refereed proceedings of the 6th International Colloquium on Theoretical Aspects of Computing, ICTAC 2009 held in Kuala Lumpur, Malaysia, in August 2009. The 17 revised full papers and 3 revised papers presented with 4 invited lectures were carefully reviewed and selected from 81 submissions. The papers address all theoretical aspects and methodological issues of computing, such as software specification, refinement, verification and testing, model checking and theorem proving, software architectures, coordination and feature interaction, integration of theories, formal and engineering methods and tools, models of concurrency, security, and mobility, parallel, distributed, and internet-based (grid) computing, real-time, embedded and hybrid systems, automata theory and formal languages, principles and semantics of languages, logics and their applications, type and category theory in computer science, case studies, theories, tools and experiments of verified systems, service-oriented architectures, as well as domain modelling and domain-specific technology.