Technical Abstract: With the recent and upcoming launches of satellite-based thermal sensors for earth observation, there is a greater opportunity to use thermal infrared (TIR) information for monitoring and managing plant ecosystem health. This chapter presents a short history of developments in TIR physics and technology, and a review of several TIR indices and models that could be useful for biosystem evaluation. Sufficient detail is provided to allow readers to compare and evaluate the indices with minimal reference to other publications. The chapter concludes with a brief discussion of the pitfalls associated with TIR measurements and algorithm implementation.