COMBINE is a file utility program developed by Jeff Hirsch of Lawrence Berkeley Laboratories for use with the DOE2.1C building energy simulation program. COMBINE merges the data arrays that are passed from the SYSTEMS simulation subprogram to the PLANTS simulation subprogram in DOE2.1C. To date, it has not received extensive publicity but it is a program that expands the applications and power of DOE2.1C and it deserves recognition as one of the most practical building energy simulation innovations of the past several years. This paper discusses the COMBINE program from the perspective of a user of DOE2.1C. COMBINE expands the limits of DOE2.1C by increasing the number of spaces that can be contained in a single building energy simulation model, by allowing explicit modeling of campus and district heating and cooling systems on a building by building basis, and by focusing attention on the plant simulation capabilities of the DOE series of building energy simulation programs. Demonstration of simulation techniques available when using COMBINE is provided through examples of DOE2.1C models at Kaiser Hospital in Panorama City, California; and California State University at Fresno (CSUF). The Kaiser Hospital model used COMBINE to expand an existing DOE2.1C model that already contained full data arrays. At CSUF, the COMBINE program was used to facilitate the explicit modeling of the entire college campus on a building by building basis. COMBINE creates new opportunities for the use of building energy simulation in the solution of complex engineering and planning problems. Use of COMBINE represents one strategy for improving the state of simulation software with a minimum of additional effort. Future developments that would improve COMBINE include the development of COMBINE for use with the LOADS subprogram and for use with microcomputer-based versions of DOE2.1.