During resin transfer molding (RTM) or structural reaction injection molding (SRIM), a low viscosity fluid is injected into a mold containing preplaced fiber reinforcement. The purpose of this paper is to present a mathematical model for the filling stage in RTM or SRIM when the viscosity and cavity thickness can be assumed to remain constant. We will show how by a transformation of variables that the equations governing RTM flow, become the same as the ones for injection molding into an empty mold when the thin gap approximation (Hele Shaw model) is made. We will also show results of simulating the filling of a typical body panel and show how the model can be used to estimate where to locate injection points and air vents. Simple algebraic equations which can be used to estimate fill times without running the computer program are given. Permeability and paste viscosity measurements are also given.