A reasonable model for the evolution of AGN is essential to estimate their contribution to cosmic backgrounds. Possible scenarios relating the evolution of individual AGN and that of the overall population are described as well as the ingredients and assumptions that are used when constructing the luminosity function. The similarity between the luminosity function and evolution of broad line AGN selected at X-ray and optical wavelengths is impressive; in both cases it is well described by luminosity evolution for a low density universe. In a critical or closed universe there is an excess of low luminosity objects at high redshift compared to pure luminosity evolution. However, the evolution is still better characterised by luminosity evolution than density evolution. The consequences for the AGN contribution to the X-ray background are described.

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