The introduction of wavelength division multiplexing into the access network is a potentially attractive alternative upgrade strategy to increasing the bit rate of passive optical networks (PON). A number of different methods have been proposed to implement a multi-wavelength PON and hence a careful assignment needs to be made of the most cost effective, both in the short term and for future increases in capacity. One key issue is the use of dynamic allocation of wavelengths, which allows the reconfiguration of the network in response to network demand. A model has been produced to simulate demand patterns on a PON, in order to quantify the benefits of dynamic assignment.