This article, which forms part of the life sciences series, examines cellular organisation in the formation of body tissues, membranes, organs and systems. Four main types of body tissue are described: epithelial, connective, muscle and nervous tissue. Each class of tissue is described in terms of its structure and function. Where appropriate, subgroups within the classifications are identified. Different membranes in the body are considered and the organisation of tissues and membranes into more complex structures, such as organs and body systems, is outlined.