Cell adhesion is the process by which cells interact and attach to neighbouring cells through specialised molecules of the cell surface. This process can occur either through direct contact between cell surfaces such as cell junctions or indirect interaction, where cells attach to surrounding extracellular matrix (ECM), a gel-like structure containing molecules released by cells into spaces between them. Cells adhesion occurs from the interactions between cell-adhesion molecules (CAMs), transmembrane proteins located on the cell surface. Cell adhesion links cells in different ways and can be involved in signal transduction for cells to detect and respond to changes in the surroundings. Other cellular processes regulated by cell adhesion include cell migration and tissue development in multicellular organisms. Alterations in cell adhesion can disrupt important cellular processes and lead to a variety of diseases, including cancer and arthritis. Cell adhesion is also essential for infectious organisms, such as bacteria or viruses, to cause diseases.

CAMs are classified into four major families: integrins, immunoglobulin (Ig) superfamily, cadherins, and selectins. Cadherins and IgSF are homophilic CAMs, as they directly bind to the same type of CAMs on another cell, while integrins and selectins are heterophilic CAMs that bind to different types of CAMs. ^{[citation needed]} Each of these adhesion molecules has a different function and recognizes different ligands. Defects in cell adhesion are usually attributable to defects in expression of CAMs.

In multicellular organisms, bindings between CAMs allow cells to adhere to one another and creates structures called cell junctions. According to their functions, the cell junctions can be classified as:

Alternatively, cell junctions can be categorised into two main types according to what interacts with the cell: cell–cell junctions, mainly mediated by cadherins, and cell–matrix junctions, mainly mediated by integrins.

Cell–cell junctions can occur in different forms. In anchoring junctions between cells such as adherens junctions and desmosomes, the main CAMs present are the cadherins. This family of CAMs are membrane proteins that mediate cell–cell adhesion through its extracellular domains and require extracellular Ca²⁺ ions to function correctly. Cadherins forms homophilic attachment between themselves, which results in cells of a similar type sticking together and can lead to selective cell adhesion, allowing vertebrate cells to assemble into organised tissues. Cadherins are essential for cell–cell adhesion and cell signalling in multicellular animals and can be separated into two types: classical cadherins and non-classical cadherins.

Adherens junctions mainly function to maintain shape of tissues and hold cells together. In adherens junctions, classical cadherins between neighbouring cells interact through their extracellular domains. Classical cadherins share a conserved calcium-sensitive region in their extracellular domains. When this region comes into contact with Ca²⁺ ions, extracellular domains of cadherins change from the inactive flexible conformation to a more rigid conformation to undergo homophilic binding. Intracellular domains of classical cadherins are also highly conserved as they bind to proteins called catenins forming catenin-cadherin complexes, which links classical cadherins to actin filaments. This association with actin filaments is essential for adherens junctions to stabilises cell–cell adhesion. Interactions with actin filaments can also promote clustering of cadherins, involved in assembly of adherens junctions, as cadherin clusters promotes actin filaments polymerisation which in turn assembles adherens junctions by binding to cadherin–catenin complexes forming at the junction.

Desmosomes are structurally similar to adherens junctions but composed of different components. Instead of classical cadherins, non-classical cadherins such as desmogleins and desmocollins act as adhesion molecules and they are linked to intermediate filaments instead of actin filaments. No catenin is present in desmosomes, as intracellular domains of desmosomal cadherins interact with desmosomal plaque proteins, which form the thick cytoplasmic plaques in desmosomes and link cadherins to intermediate filaments. Desmosomes provides strength and resistance to mechanical stress by unloading forces onto the flexible but resilient intermediate filaments, something that cannot occur with the rigid actin filaments. This makes desmosomes important in tissues that encounter high levels of mechanical stress, such as heart muscle and epithelia, and explains why it appears frequently in these types of tissues.

Tight junctions are normally present in epithelial and endothelial tissues, where they seal gaps and regulate paracellular transport of solutes and extracellular fluids in these tissues that function as barriers. Tight junction is formed by transmembrane proteins, including claudins, occludins and tricellulins, that bind closely to each other on adjacent membranes in a homophilic manner. Similar to anchoring junctions, intracellular domains of these tight junction proteins are bound with scaffold proteins that keep these proteins in clusters and link them to actin filaments in order to maintain structure of the tight junction. Claudins, essential for formation of tight junctions, form paracellular pores which allow selective passage of specific ions across tight junctions making the barrier selectively permeable.