Source parameters of foreshocks and aftershocks of 27 large (M ≧ 5.5) earthquake sequences occurring within the western cordillera of the United States between 1915 and 1988 have been used to examine the properties of foreshocks and aftershocks and to determine how foreshocks and aftershocks relate to faulting properties of their main shocks, to physiographic province, and to heat flow. This study examines only the largest foreshocks and aftershocks occurring within 2 months and 40 km of the main shocks. Results of this study show that nearly 60 per cent of western cordilleran earthquakes have foreshocks and that 58 per cent of the foreshocks occurred within 24 hours of their main shocks. Most aftershocks appear to be occurring on a fault plane with the same or similar orientation to the fault that ruptured in the main shock at or slightly above the main shock depth. The average magnitude difference between main shocks and their largest aftershocks is 1.03 ± 0.47. The difference between main shock and aftershock magnitude appears to decrease with increasing heat flow. Sixty percent of the aftershocks occurred within 24 hours of their main shocks, but nearly one third occurred more than 1 week after their main shocks.