Although the primary use of RXWINDOW will be for prescribed fire planning, it has applications in other fire management activities where there is a need to relate potential fire behavior to environmental conditions. For example, RXWINDOW can be used on a wildfire to determine appropriate conditions for burnout and backfire. RXWINDOW is a program in the BEHAVE fire behavior prediction and fuel modeling system (Andrews 1986; Andrews and Chase 1989; Burgan and Rothermel 1984). It reverses some of the processes that are available elsewhere in BEHAVE. In RXWINDOW, you define acceptable ranges of fire behavior and the program determines the appropriate combinations of fuel moisture and wind. For example, if you specify a range of flame lengths, RXWINDOW will give you acceptable limits for dead and live fuel moisture and for windspeed and direction. You can also specify ranges for desired rate of spread, intensity, or the first-order fire effects, scorch height, or tree mortality. For simplicity in this paper we refer to all of these as fire behavior variables. The program does not include a calculation of fuel moisture from temperature and relative humidity. The need for a program such as RXWINDOW is evidenced by the fact that many fire managers have been using BEHAVE in establishing burning prescriptions (Andrews and Bradshaw 1987; Doren and others 1987). Although the mathematical models in BEHAVE include some assumptions that limit their use for certain prescribed fire applications, skilled users are able to use the prediction with some confidence. In this paper we describe the approach of basing prescription windows on fire behavior and explain where the RXWINDOW program fits into the prescribed fire planning process. The assumptions and limitations of the predictive models used in the program are discussed. Operation of the program is described, and an appendix contains a detailed annotated example run. We assume that you are familiar with the BEHAVE system. References will be made to previous BEHAVE documents rather than repeating all of that information here.