THEMIS is a tool to facilitate the design, development, and analysis of decentralized monitoring algorithms; developed using Java and AspectJ. It consists of a library and command-line tools. THEMIS provides an API, data structures and measures for decentralized monitoring. These building blocks can be reused or extended to modify existing algorithms, design new more intricate algorithms, and elaborate new approaches to assess existing algorithms. We illustrate the usage of THEMIS by comparing two variants of a monitoring algorithm.