A complex and diverse community of bacteria, fungi, viruses and mites densely colonises the human skin. The entire collection of all microbial genes in a community of a given habitat is usually referred to as the microbiome and will for the sake of consistency be used throughout this chapter. The human skin microbiome is individually unique in composition and shaped by several factors in close relation to the skin barrier. Current research strives to establish common characteristics of the healthy skin microbiome from which to detect alterations associated with skin diseases. Studying the human skin microbiome composition may be valuable in elucidating the aetiology of various, common skin diseases with the hope of therapeutic approaches targeting the skin microbiome. Based on current knowledge, this chapter gives an introduction to the human skin microbiome, primarily the bacterial, and explains factors of importance for the healthy and diseased skin microbiome composition.