The Advanced Sensor Development Laboratory (ASDL) at the Stennis Space Center develops, maintains and calibrates remote sensing instruments for the National Aeronautics & Space Administration. To perform system design trade-offs, analysis, and establish system parameters, ASDL has developed a software package for analytical simulation of sensor systems. This package called 'Analytical Tools for Thermal InfraRed Engineering'--ATTIRE, simulates the various components of a sensor system. The software allows each subsystem of the sensor to be analyzed independently for its performance. These performance parameters are then integrated to obtain system level information such as SNR, NER, NETD etc. This paper describes the uses of the package and the physics that were used to derive the performance parameters. In addition, ATTIRE can be used as a tutorial for understanding the distribution of thermal flux or solar irradiance over selected bandwidths of the spectrum. This spectrally distributed incident flux can then be analyzed as it propagates through the subsystems that constitute the entire sensor. ATTIRE provides a variety of functions ranging from plotting black-body curves for varying bandwidths and computing the integral flux, to performing transfer function analysis of the sensor system. The package runs from a menu- driven interface in a PC-DOS environment. Each sub-system of the sensor is represented by windows and icons. A user-friendly mouse-controlled point-and-click interface allows the user to simulate various aspects of a sensor. The package can simulate a theoretical sensor system. Trade-off studies can be easily done by changing the appropriate parameters and monitoring the effect of the system performance. The package can provide plots of system performance versus any system parameter. A parameter (such as the entrance aperture of the optics) could be varied and its effect on another parameter (e.g., NETD) can be plotted. A third parameter (e.g., the obscuration) could be varied for each plot and several plots obtained on the same graph. The menu for such 'Y vs X plots for different values of Z' currently contains various such options. The package also allows the user to create customized work-sheets of the simulated system and save the analysis for interface with or retrieval to other packages. The emissivity, atmospheric transmission and the optical transmission default as constants over the specified spectral bandwidths. There is an option for making these parameters spectrally variable. If more than one of the above-mentioned three parameters are spectrally variable, then it is possible that the upper and lower wavelength values as well as the resolution of the wavelength array may not be the same for all three arrays. The package performs an interpolation of the data to smooth out the curves and then projects them onto a common wavelength array for all the parameters.