This book constitutes the refereed proceedings of the 15th International Workshop of Descriptional Complexity of Formal Systems, DCFS 2013, held in London, ON, Canada, in July 2013. The 22 revised full papers presented together with 4 invited papers were carefully reviewed and selected from 46 submissions. The topics covered are automata, grammars, languages and other formal systems; various modes of operations and complexity measures; co-operating systems; succinctness of description of objects, state-explosion-like phenomena; circuit complexity of Boolean functions and related measures; size complexity and structural complexity of formal systems; trade-offs between computational models and mode of operation; applications of formal systems; for instance in software and hardware testing, in dialogue systems, in systems modeling or in modeling natural languages; and their complexity constraints; size or structural complexity of formal systems for modeling natural languages; complexity aspects related to the combinatorics of words; descriptional complexity in resource-bounded or structure-bounded environments; structural complexity as related to descriptional complexity; frontiers between decidability and undecidability; universality and reversibility; nature-motivated (bio-inspired) architectures and unconventional models of computing; Kolmogorov-Chaitin complexity, algorithmic information.