The behavioural mechanisms that may contribute to the accumulation of leucocytes (e.g. in inflammatory sites) are reviewed. Almost all of the neutrophils and monocytes of blood are motile and capable of migrating into tissues, but many blood lymphocytes are non-motile. They can however be induced to acquire locomotor capacity by culture with growth activators such as anti-CD3 antibodies. Resting lymphocytes become motile after entering the G1 phase of the cell cycle. Chemotaxis is well-studied in neutrophil leucocytes and is an efficient mechanism for cell accumulation. Possible mechanisms by which leucocytes respond to attractant gradients are discussed. Non-chemotactic mechanisms may also lead to cell accumulation, and evidence is reviewed that shows that an adhesive trapping mechanism causes leucocytes that cross an adhesion boundary to accumulate in the absence of a chemotactic gradient. Leucocytes migrating through aligned connective tissue show contact guidance in the axis of alignment of the tissue. Contact guidance can reinforce or hinder the response of neutrophils to chemotactic gradients.