The RETRAN-02 transient thermal-hydraulic analysis program has found wide acceptance by the nuclear utility industry for performing analyses of operational transients and small-break loss-of-coolant accidents. There are, however, known limitations with RETRAN-02 and the Electric Power Research Institute has sponsored the development of RETRAN-03 to address these limitations. The major objectives of the development program are (a) to extend the range of analyses that can be performed with RETRAN, (b) to make the code more dependable and faster running, and (c) to have a more transportable code. Summaries of the RETRAN-02 models that have been modified for RETRAN-03, new models that have been added, and the new semi-implicit steady-state and dynamic solution methods are presented. The primary development task associated with improving the transportability of the code dealt with implementing the IBM and CDC environmental libraries (assembly language) using a common FORTRAN 77 source. The salient results of this work are discussed.