A new flow boiling model and generalized correlation are presented for evaporation inside of horizontal microfin tubes. The enhancement model incorporates the effects of the microfins on liquid-phase convection and nucleate boiling. The method predicts microfin test data for R-134a and R-123 over the mass velocity range from 100-500 kg/m2s, vapor qualities from 15- 85% and heat fluxes from 2-47 kW/M2. The model is the first general type of method to become available and can be applied to any 2-dimensional microfin geometry and fluid of interest, without resort to fluid specific or microfin specific empirical constats. More work is required to compare the model to independent test data and other fluids and microfin geometries.