1. A method of tracking objects in a series of n-D images that have objects appearing in a background comprising using a probabilistic model of the appearance of the objects and of the appearance of the background in the images, and using an evaluation of whether particular pixels in the images are a part of an object or a part of the background, that evaluation comprising determining the posterior model probabilities that a particular pixel or group of pixels belongs to an object or to the background, and further comprising marginalising over these object/background membership probabilities to yield a function of the pose parameters of the objects, where at least the object/background membership is adjudged to be a nuisance parameter and marginalised out.