A computational model has been developed to analyze the transient and steady-state performance of flat heat pipes and assess their performance under different operating and geometric parameters, in order to arrive at optimal designs. The model assumes two-dimensional fields for flow and heat transfer and solves the governing differential equations using a finite-difference approach. The wick region of the heat pipe is analyzed using transport equations for a porous medium. The influence of axial heat conduction along the wall, as well as the energy transport in the wick, on the velocity and temperature distributions is examined. The overall performance of the heat pipe is quantified by calculating an effective thermal conductance from the heat input and the temperature drop along the heat pipe wall. Parametric studies are conducted using the model to investigate the dependence of the heat pipe performance on the heat input at the evaporator, the containing wall thickness, and the porosity of the wick.