NOAA-18 Advanced Microwave Sounding Unit-A (AMSUA), Microwave Humidity Sounder (MHS), and Advanced Very High Resolution Radiometer/3 (AVHRR/3), along with collocated CloudSat data under nonprecipitation conditions, are used to validate the accuracy of the Community Radiative Transfer Model (CRTM). The observed brightness temperatures (BTs) from NOAA-18 instruments are compared to those simulated by the CRTM using the inputs of CloudSat retrieved hydrometeor profiles. The forward model biases are computed for various cloudy conditions, which are required for the assimilation of satellite cloudy radiances in operational forecast systems. Simulated BTs under nonprecipitation, cloudy conditions are averaged in space to account for the cloud inhomogeneity within the sensors’ fields of view. The simulated and observed BT fields, BT distributions, and BT difference distributions show good agreement for all microwave channels. Simulations under clear skies in general have low biases and standard deviation errors, and these errors are only marginally increased under cloudy conditions for microwave channels. For AVHRR channels 4 and 5, the biases and standard deviation errors are low and very accurate for clear and water cloud conditions. However, there are larger standard deviation errors under cirrus and mixed-phase cloud conditions for those channels. The spatial averaging method significantly reduced the standard deviation errors under cloudy conditions. In this study, we have validated the CRTM modules (gaseous absorption model, cloud absorption and scattering model, and surface emissivity models over ocean) that generate optical properties of the atmosphere and surface in the microwave and thermal infrared spectral region.