A scheme is presented for dealing with the full range of fire behavior in conifer forests. It is based on empirical data from fires in Canadian forests plus a theory to describe the physical conditions for the transition from surface to crown fire. In its ideal form, the model consists of two distinct equations for spread rate in a given forest, one each for surface and crown fire, plus the rules for the transition upward or downward between them. For lack of enough data in some cases, a compromise has been devised that distinguishes between surface and crown fires, but docs not provide for complete separation of the spread-rate functions. The scheme will form part of the forthcoming Canadian Forest Fire Behavior Prediction (FBP) system.