For the past several years, considerable time and effort have been expended on the various aspects of a micro-calorimetry system. Due to the complexity and interdisciplinary nature of the work, the first model did not perform to expectations. Several problem areas were identified and led to subsequent projects (e.g., 71-78-167E, Monolithic Thermopile Design and 71-80-144M, R-C Analysis of Micro-Calorimetric Transient Response). The review and redesign led to the achievement of heat resolution of 300n watts and long-term stability of plus or minus 200nw (rms). A list of performance criteria for a differential calorimeter has been developed and the new sensor design is being evaluated in a new calorimeter.