We present the MIT Finite-State Transducer Toolkit and briefly describe research that it has benefitted. The toolkit is a collection of command-line tools and associated C++ API for manipulating finite-state transducers (FSTs) and acceptors (FSAs) and has been designed to enable research through its flexibility, yet remain efficient enough to aid real-world computationally demanding applications such as automatic speech recognition. The toolkit supports the construction, combination, optimization, and training of weighted FSTs and FSAs, and as such is useful in many areas of human language technology.