The innate immune system is rapidly activated in response to infection and injury. It is a generic rather than pathogen-specific response that recruits immune cells, promotes inflammation, and mobilizes the adaptive immune system. Excessive or chronic inflammation may cause tissue damage, so a careful balance is required to restore homeostasis. Written and edited by experts in the field, this collection from Cold Spring Harbor Perspectives in Biology reviews the cellular and molecular mechanisms involved in innate immunity and all types of inflammation. The contributors examine the cell types that make up the innate immune system, their use of pattern recognition receptors (e.g., Toll-like receptors) to identify pathogens and damaged tissues, and how they trigger signaling pathways that culminate in inflammation, pathogen destruction, and tissue repair. The numerous chemical signals and factors involved in innate immunity and inflammation are described, as are those that keep inflammation in check. The authors also discuss the diseases that can result when these processes go awry, such as rheumatoid arthritis and cancer. This volume is therefore a valuable reference for all immunologists, cell biologists, and medical scientists wishing to understand these protective processes and their implications for human health and disease.