The Hydrologic Evaluation of Landfill Performance (HELP) program was developed to facilitate rapid, economical estimation of the amounts of surface runoff, subsurface drainage, and leachate that may be expected to result from the operation of a wide variety of possible landfill designs. The program models the effects of hydrologic processes including precipitation, surface storage, runoff, infiltration, percolation, evapotranspiration, soil moisture storage, and lateral drainage using a quasi-two-dimensional approach. In this document, the theories and assumption upon which the HELP model is based, the solution techniques employed, and the internal logic of the computer program are presented and discussed in detail.