Minimum Resolvable Temperature Difference (MRTD) is an invaluable tool in designing and testing infrared sensors and is to a lesser extent valuable in predicting field performance. The history of MRTD use is briefly reviewed and some shortcomings of MRTD are noted especially in the discrepancies between predicted and tested performance in the laboratory. Newer developments in infrared technology have altered the form of sensors from the FLIR concepts originally treated by the existing MRTD models. Infrared Linescanners, Focal Plane Arrays and other second generation sensors must be included. Some suggestions are given for modifying existing MRTD models to include these other sensors. A concept of "objective MRTD" is presented to allow use of MRTD in systems such as Automatic Target Screeners (ATS) and Automated Test Equipment (ATE) where the human operator is temporarily bypassed.