A one dimensional (1-D) air gap membrane distillation (AGMD) model for flat sheet type modules was developed. This model is based on mathematical equations that describe the heat and mass transfer mechanisms of a single pass AGMD process. The validity of the model was tested against several experimental measurements made at different operating conditions. For instance, the model flux prediction was compared to the flux measured at five different feed temperatures, two different feed water salinities, three different air gap widths and two membranes with different average pore sizes. This comparison showed that the model predictions agree well with the experimental data. Some of the potential uses of this model include analyzing the most dominant mass transfer resistance within AGMD module, calculating the thermal efficiency of AGMD process and guiding the design of its future scale up.