This study has evaluated numerous analytical models for countercurrent capillary imbibition for use as matrix-fracture transfer functions in waterflood models for naturally fractured reservoirs. Relationships between the various models were developed and were used to reformulate the models into common terms so they could be compared. The models were then validated against experimental data; both previously published data and new data obtained in this study were used. From this analysis, the strengths and weaknesses of each model were determined. The square-root model was the best model for the early-time period when the finite size of the matrix block did not impact imbibition. A modified power-law model provided the best model for the late-time period when the finite size of the matrix block did impact imbibition. An exponential model provided the best overall model for imbibition through both time periods, although it was not accurate for the very early times.