This book constitutes the refereed proceedings of the 21st International Conference on Analytical and Stochastic Modelling Techniques and Applications, ASMTA 2014, held in Budapest, Hungary, in June/July 2014. The 18 papers presented were carefully reviewed and selected from 27 submissions. The papers discuss the latest developments in analytical, numerical and simulation algorithms for stochastic systems, including Markov processes, queueing networks, stochastic Petri nets, process algebras, game theory, etc.