Integrins are cell adhesion receptors that play important roles in developmental and pathological processes. They are composed of 24 αβ heterodimeric members that mediate cell attachment to the extracellular matrix (ECM) and participate in specialized cell-cell interactions. Only a subset of integrins (8 out of 24) recognize the RGD sequence in native ligands. In some ECM molecules, such as collagen and certain laminin isoforms, the RGD sequences are exposed upon denaturation or proteolytic cleavage, allowing cells to bind these ligands via RGD-binding receptors. Proteolytic cleavage of ECM proteins may also generate fragments with novel biological activity, such as endostatin, tumstatin, and endorepellin. Nine integrin α chains contain an αI domain, including the collagen-binding integrins α1β1, α2β1, α10β1, and α11β1. The collagen-binding integrins recognize the triple-helical GFOGER sequence in major collagens, but their ability to recognize these sequences in vivo is dependent on the fibrillar status and accessibility of the interactive domains in the fibrillar collagens. The current review summarizes some basic facts about the integrin family, including a historical perspective, their structure, and their ligand-binding properties. Integrins are cell adhesion receptors that are evolutionary old. Despite their long history, they have only been characterized at the molecular level for approximately 25 years. During this period, a large number of articles on the ever-increasing intricacies of integrin action has been published. To summarize this large amount of data on integrins "at a glance" is almost an impossible task. In the current review, we have however attempted to provide some of the basic facts about integrins. Recent excellent reviews on various aspects of integrins structure and function will be referred to in the text. One reason for the difficulties encountered when trying to characterize the integrin family is that many of their ligands are large multi-adhesive extracellular matrix (ECM) molecules that, in addition to binding integrins, bind other proteins including ECM molecules, growth factors, cytokines, and matrix-degrading proteases. One successful approach that was instrumental in the identification of integrins took advantage of antibodies that blocked cell adhesion. In another approach, the mapping of the minimal cell adhesion site in fibronectin to the RGDS sequence gave rise to affinity chromatography protocols with increased specificity. In these affinity protocols, the optimization of ion composition in the purification buffers was essential and resulted in the empirical finding that manganese ions increased integrin affinity. In 1986, the antibody approach led to the expression cloning of cDNA encoding the chick integrin β1 subunit. The name “integrin” was given to denote the importance of these receptors for maintaining the integrity of the cytoskeletal-Integrins are cell adhesion receptors that play important roles in developmental and pathological processes. They are composed of 24 αβ heterodimeric members that mediate cell attachment to the extracellular matrix (ECM) and participate in specialized cell-cell interactions. Only a subset of integrins (8 out of 24) recognize the RGD sequence in native ligands. In some ECM molecules, such as collagen and certain laminin isoforms, the RGD sequences are exposed upon denaturation or proteolytic cleavage, allowing cells to bind these ligands via RGD-binding receptors. Proteolytic cleavage of ECM proteins may also generate fragments with novel biological activity, such as endostatin, tumstatin, and endorepellin. Nine integrin α chains contain an αI domain, including the collagen-binding integrins α1β1, α2β1, α10β1, and α11β1. The collagen-binding integrins recognize the triple-helical GFOGER sequence in major collagens, but their ability to recognize these sequences in vivo is dependent on the fibrillar status and accessibility of the interactive domains in the fibrillar collagens. The current review summarizes some basic facts about the integrin family, including a historical perspective, their structure, and their ligand-binding properties. Integrins are cell adhesion receptors that are evolutionary old. Despite their long history, they have only been characterized at the molecular level for approximately 25 years. During this period, a large number of articles on the ever-increasing intricacies of integrin action has been published. To summarize this large amount of data on integrins "at a glance" is almost an impossible task. In the current review, we have however attempted to provide some of the basic facts about integrins. Recent excellent reviews on various aspects of integrins structure and function will be referred to in the text. One reason for the difficulties encountered when trying to characterize the integrin family is that many of their ligands are large multi-adhesive extracellular matrix (ECM) molecules that, in addition to binding integrins, bind other proteins including ECM molecules, growth factors, cytokines, and matrix-degrading proteases. One successful approach that was instrumental in the identification of integrins took advantage of antibodies that blocked cell adhesion. In another approach, the mapping of the minimal cell adhesion site in fibronectin to the RGDS sequence gave rise to affinity chromatography protocols with increased specificity. In these affinity protocols, the optimization of ion composition in the purification buffers was essential and resulted in the empirical finding that manganese ions increased integrin affinity. In 1986, the antibody approach led to the expression cloning of cDNA encoding the chick integrin β1 subunit. The name “integrin” was given to denote the importance of these receptors for maintaining the integrity of the cytoskeletal-