  We propose a definition for the P\'olya number of continuous-time quantum
walks to characterize their recurrence properties. The definition involves a
series of measurements on the system, each carried out on a different member
from an ensemble in order to minimize the disturbance caused by it. We examine
various graphs, including the ring, the line, higher dimensional integer
lattices and a number of other graphs and calculate their P\'olya number. For
the timing of the measurements a Poisson process as well as regular timing are
discussed. We find that the speed of decay for the probability at the origin is
the key for recurrence.
