This report presents a modular three-dimensional finite-difference ground-water flow model developed by Michael G. McDonald and Arlen W. Harbaugh. The model simulates three-dimensional groundwater flow and is based on a modular program structure that allows for flexibility and ease of modification. The program is written in FORTRAN '66 and can run on various computers with minimal changes. It includes a main program and a series of independent subroutines called modules, grouped into packages that handle specific hydrologic features or solution methods. The model uses a block-centered finite-difference approach to simulate groundwater flow within an aquifer. It can simulate confined, unconfined, or a combination of both types of aquifers. The model can also simulate flow from external stresses such as well discharge, areal recharge, evapotranspiration, flow to drains, and flow through riverbeds. The finite-difference equations can be solved using either the Strongly Implicit Procedure (SIP) or Slice-Successive Overrelaxation (SSOR) methods. The program is designed to be flexible, with input and output systems that allow users to select various output options. The model includes several packages for simulating different hydrologic features, such as wells, rivers, drains, evapotranspiration, and general-head boundaries. Each package is described in a separate chapter, including conceptualization, input instructions, sample input, and module documentation. The report explains the mathematical model used to describe groundwater flow, including the derivation of the finite-difference equation. It also discusses the program's structure, including the use of a modular approach, the discretization convention, and the input and output systems. The report provides detailed descriptions of each module, including their functions, flow charts, and variable listings. It also includes appendices with additional information on program portability, space requirements, and sample problems.This report presents a modular three-dimensional finite-difference ground-water flow model developed by Michael G. McDonald and Arlen W. Harbaugh. The model simulates three-dimensional groundwater flow and is based on a modular program structure that allows for flexibility and ease of modification. The program is written in FORTRAN '66 and can run on various computers with minimal changes. It includes a main program and a series of independent subroutines called modules, grouped into packages that handle specific hydrologic features or solution methods. The model uses a block-centered finite-difference approach to simulate groundwater flow within an aquifer. It can simulate confined, unconfined, or a combination of both types of aquifers. The model can also simulate flow from external stresses such as well discharge, areal recharge, evapotranspiration, flow to drains, and flow through riverbeds. The finite-difference equations can be solved using either the Strongly Implicit Procedure (SIP) or Slice-Successive Overrelaxation (SSOR) methods. The program is designed to be flexible, with input and output systems that allow users to select various output options. The model includes several packages for simulating different hydrologic features, such as wells, rivers, drains, evapotranspiration, and general-head boundaries. Each package is described in a separate chapter, including conceptualization, input instructions, sample input, and module documentation. The report explains the mathematical model used to describe groundwater flow, including the derivation of the finite-difference equation. It also discusses the program's structure, including the use of a modular approach, the discretization convention, and the input and output systems. The report provides detailed descriptions of each module, including their functions, flow charts, and variable listings. It also includes appendices with additional information on program portability, space requirements, and sample problems.