  We introduce a canonical method for pair production by electromagnetic
fields. The canonical method in the space-dependent gauge provides
pair-production rate even for inhomogeneous fields. Further, the instanton
action including all corrections leads to an accurate formula for the
pair-production rate. We discuss various aspects of the canonical method and
clarify terminology for pair production. We study pair production by charged
black holes first by finding states of the field equation that describe pair
production and then by applying the canonical method.
