BACKGROUND Tissue expansion is a concept based on the skin's natural ability to stretch in response to an underlying force. OBJECTIVE The purpose of this article is to review the histological and physiological changes that occur within the soft tissue and underlying structures during tissue expansion. METHODS An extensive search of the literature reviewing these changes is summarized herein. RESULTS Conventional tissue expansion may result in epidermal hypertrophy, decreased dermal, muscle, and adipose thickness, and bone resorption. A vascular capsule and angiogenesis provides a highly vascular flap and improves flap viability. Few soft tissue changes occur during rapid tissue expansion. The ability of the skin to increase in surface area during conventional tissue expansion is primarily because of biological tissue creep. Rapid expansion may result from mechanical tissue creep. CONCLUSIONS Many soft tissue changes occur during tissue expansion. Most of these changes return to the pre-expansion state over time following discontinuation of the expansion process.