We present a novel spatio-temporal master equation (ME) for describing
the evolution of optical fields in laser cavities. Our ME introduces
a new type of propagation operator explicitly dependent on the ABCD el
ements of the cavity. We derive this and show that it correctly reprod
uces the cavity mode structure in the linear limit. We apply our ME to
the problem of Kerr lens mode-locking (KLM) and show that our numeric
al results, in one dimension (x), are in excellent agreement with thos
e found using the more conventional Huygens' integral method. Dispersi
on and other fast-time effects are then added to give a full spatio-te
mporal ME. Again this is applied to KLM and we show that stable solito
n-like pulses result.