Spatial distribution of ecological populations is very rarely homogeneous. On the contrary, patches of high population density quite often alternate with patches of low density or even with uninhabited areas. It can occur on different spatial scales and in various environment and ecosystem types. In population biology and spatial ecology, this phenomenon may come under a variety of names such as plant/animal grouping or aggregation, patchiness, etc. In some cases (e.g., see Section 9.3), the corresponding spatial structure exhibits prominent regularity, even if it might be blurred by environmental noise. Much more often, however, populations are distributed irregularly, without any detectable trace of order.