Diffraction caused by finite apertures in long baseline interferometers leads to a significant loss of signal, decreased fringe visibility, and a phase shift in the interference pattern. Formulas are developed for the general case of an interferometer with unequal path lengths for monochromatic plane wave illumination. Numerical results are given for a range of path lengths. The consequences of diffraction on the performance of long path instruments, particularly with reference to a long baseline Michelson stellar interferometer, are discussed.