Streamflow is not measured at every location in a stream network. Yet hydrologists, State and local agencies, and the general public still seek to know streamflow characteristics, such as mean annual flow or flood flows with different exceed- ance probabilities, at ungaged basins. The goals of this guide are to introduce and familiarize the user with the weighted- multiple-linear regression (WREG) program, and to also provide the theoretical background for program features. The program is intended to be used to develop a regional estima- tion equation for streamflow characteristics that can be applied at an ungaged basin, or to improve the corresponding estimate at continuous-record streamflow gages (henceforth referred to as simply gages) with short records. The regional estimation equation results from a multiple-linear regression that relates the observable basin characteristics, such as drainage area, to streamflow characteristics (for example, Thomas and Benson, 1970; Giese and Mason, 1993; Ries and Friesz, 2000; Eng and others, 2005; Eng and others, 2007a; Eng and others, 2007b; Kenney and others, 2007; Funkhouser and others, 2008).