The most common way of generating correlated photons is by parametric down-
conversion in a nonlinear crystal in free space. However, a drawback of thi
s technique is the extremely low photon flux available. We present a simple
analysis which shows that cavity-enhanced parametric down-conversion, usin
g an arrangement similar to an optical parametric oscillator and operated w
ell below threshold, can yield a substantially higher photon flux without s
ignificantly degrading the correlation between photons.