Often it is appropriate to consider the analysis of the geo-referenced distribution of more than one disease. For example, the focus may be on a group of diseases with similar etiology in an epidemiological study. Another example, in the context of public health, could be the examination of the general health status of a region (possibly following a cluster alarm signal). In the latter case a range of disease types might be considered to ﬁnd out if any show signs of unusual risk variation. In some cases, the focus is on the spatial distribution of the vector of diseases (for example relative risk estimation for the vector). In other cases the diseases are to be contrasted or correlation between their spatial distributions are to be considered. In this chapter the focus will be on relative risk estimation and modeling of risk both in terms of correlation between diseases and in terms of comparison. The simplest situation where such analysis can be considered is where two diseases are to be examined.