This paper introduces the MultiAgent Decision Process software toolbox, an open source C++ library for decision-theoretic planning under uncertainty in multiagent systems. It provides support for several multiagent models, such as POSGs, Dec-POMDPs and MMDPs. The toolbox aims to reduce development time for planning algorithms and to provide a benchmarking platform by providing a number of commonly used problem descriptions. It features a parser for a text-based ﬁle format for discrete Dec-POMDPs, shared functionality for planning algorithms, as well as the implementation of several Dec-POMDP planners. We describe design goals and architecture of the toolbox, and provide an overview of its functionality, illustrated by some usage examples. Finally we report on current and future work.