First and second generation of infrared (IR) systems are characterized by their variation in the measurement of the Minimum Resolvable Temperature Difference (MRTD). This paper is directed toward the basic measurement of MRTD for both generations of IR systems. Concepts and theory behind the measurment were addressed. The MRTD used for modern systems production and development was introduced. A practical measurement procedure for laboratory imaging scanner type system results and data analysis are also provided. A discussion of the challenges of evaluating new systems such as staring arrays and second generation digital IR systems and the requirements for both horizontal and vertical MRTD measurements are also provided.