The process whereby cells interact and attach to other cells or to inanimate surfaces. Cell adhesion is mediated by cell surface proteins and associated macromolecules. It forms the physical basis of multicellularity, morphogenesis and embryonic development, tissue integrity, host–pathogen interactions, immune system function, and the ecological integration of microbial communities. In addition to attaching cells to one another, cell adhesion systems act to coordinate a wide range of cellular behaviors. In animal cells, sites of adhesion serve to organize the system of intracellular protein filaments, known as the cytoskeleton, responsible for cell shape and movement. Adhesive interactions are critical for juxtacrine signaling, by which neighboring cells communicate. Juxtacrine signals regulate gene expression and cell behavior and determine whether a cell survives, divides, differentiates, or undergoes programmed cell death (apoptosis).