The seismicity of western Canada has been studied for the period 1899–1975. The quality of the data collected improved through this period as the number of recording stations increased and the location and analysis methods developed, but significant uncertainties and biases remain. Although these restrictions limit detailed correlation of seismic events with specific tectonic features, in general the most active earthquake areas correspond to the boundaries between the major lithospheric plates. These are the Queen Charlotte – Fairweather fault system (Pacific–America plates), the offshore ridge-fracture zone system (Pacific – Juan de Fuca plates), and the Vancouver Island – Puget Sound region (Juan de Fuca – America plates). Strain release calculations show that most seismic energy is released along the Queen Charlotte – Fairweather fault system and that at present a significant accumulation of strain may be available for release as earthquakes in the Vancouver Island – Puget Sound area. Except for the absence of thrust earthquakes along the apparently converging margin, focal mechanisms are in good agreement with the postulated plate motions. The b values in the frequency–magnitude recurrence relation for different areas within the region range from 0.65 to 0.82.