This publication contains documentation for the PRZM-3 model. PRZM-3 is the most recent version of a modeling system that links two subordinate models - PRZM and VADOFT - in order to predict pesticide transport and transformation down through the crop root and unsaturated soil zones. Enhancements to Release 3.0 reported herein include algorithms that also enable modeling of the nitrogen cycle soil kinetic processes, with the ability to track nitrogen discharges from a septic tank into the soil environment and its subsequent movement to groundwater. Additional included enhancements enable better simulation of physicochemical processes, increased flexibility in representing agronomic practices, and improved post-processing and data interpretation aids. PRZM is a one-dimensional, finite-difference model that accounts for pesticide and nitrogen fate in the crop root zone. PRZM-3 includes modeling capabilities for such phenomena as soil temperature simulation, volatilization and vapor phase transport in soils, irrigation simulation, microbial transformation, and a method of characteristics (MOC) algorithm to eliminate numerical dispersion. PRZM is capable of simulating the transport and the transformation of a given parent compound, and at most as two daughter species. VADOFT is a one-dimensional, finite-element code that solves the Richard's equation for flow in the unsaturated zone. The user can use constitutive relationships between pressure, water content, and hydraulic conductivity to solve the flow equations. VADOFT can simulate the fate of two parent compounds, each with two daughter products. The PRZM and VADOFT codes are linked together with the aid of a flexible execution supervisor that allows the user to build loading models tailored to the user's site-specific situations. In order to perform probability-based exposure assessments, the code is also equipped with a Monte Carlo pre- and post-processor.