Abstract Updated versions of the Tangent Linear Model (TLM) and adjoint of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) have been developed and are now available to the meteorological community. The previous version of the MM5 TLM and adjoint were designed for single-processor computer architectures, based on version 1 of MM5, and were hand coded, which made it difficult to maintain up-to-date versions of the TLM and the adjoint as MM5 evolved. The new TLM and adjoint are based on version 3 of MM5 and run efficiently on multiple-processor computers. The TLM and adjoint were developed with the aid of the Tangent Linear and Adjoint Model Compiler (TAMC) automatic code generator. While some manual intervention is still necessary, the use of the automatic code generator can significantly speed code development and lower code maintenance costs. The new TLM and adjoint contain most of the physics packages and observation operators that were available in the MM5 version 1 TLM and adjoint. The new adjoint has been combined with the MM5 version 3 nonlinear model and an updated minimization module in a four-dimensional variational data assimilation analysis configuration. Accuracy of the new TLM and adjoint has been verified by individual unit and system tests as well as comparisons with the adjoint from MM5 version 1. Timing tests showed substantial decreases in time to solution when increasing the number of processors devoted to the problem.