A computer program called the Flow and Head Boundary Package (FHB1) was developed for the U.S. Geological Survey three-dimensional finite-difference modular ground-water flow model, commonly referred to as MODFLOW. FHB1 allows MODFLOW users to specify flow or head boundary conditions that vary at times other than starting and ending times of stress periods and associated time steps. Values of flow and (or) head at each time step are calculated by linear interpolation of user- specified values. The ability to assign variable flow and head conditions defined at times not corresponding with the model stress periods allows greater flexibility in simulating natural geohydrologic systems and, at the same time, improves the efficiency of the methods used to represent these systems. The package also provides a way to apply specified-flow and specified-head boundaries in embedded, or nested, smaller-scale models using flow and (or) head values from larger-scale models. Using FHB1, the two models can have different simulation stress periods and time steps. Specification of variable-flow pumped wells in ground-water models is another example application.