Lymphocytes adhere to other cells and extracellular matrix in the process of immunological recognition and lymphocyte recirculation. This review focuses on regulation of lymphocyte adhesion and the use of adhesion mechanisms by lymphocytes to obtain information about their immediate environment. The CD2 and LFA-1 adhesion receptors appear to have distinct roles in the regulation of adhesion and modulation of T lymphocyte activation. Adhesion mediated by interaction of CD2 with LFA-3 is dramatically altered by surface charge and adhesion receptor density in such a way that this pathway is latent in resting T lymphocytes but becomes active over a period of hours following T-cell activation. CD2 ligation can mediate or enhance T-cell activation, suggesting that signals from CD2/LFA-3 adhesive interactions are integrated with signals from the T-cell antigen receptor during immunological recognition. A model for the role of LFA-3 lateral diffusion in adhesion is presented, based on the lateral diffusion of different LFA-3 forms in glass supported planar membranes. Interaction of LFA-1 with ICAMs is also regulated by cell activation but in a different way than in interaction of CD2 with LFA-3. LFA-1 avidity for ICAMs is transiently increased by T-cell activation over a period of minutes. Cycles of avidity change are also observed for other T lymphocyte integrins which bind to extracellular matrix components. We propose that integrin avidity cycles may have an important role in the interconnected phenomena of locomotion, initial cell-cell adhesion, and cell-cell deadhesion. Recent observations on recirculation of T lymphocyte subpopulations are discussed in the context of general lessons learned from study of the CD2/LFA-3 and LFA-1/ICAM adhesion mechanisms.