Membrane rafts are microdomains involved in a number of biologically important processes, including immunoreceptor signalling. Among the functionally important protein components of these microdomains are transmembrane adaptor proteins, containing in their intracellular domains tyrosine residues that can be phosphorylated and bind other cytoplasmic signalling proteins. The most important leukocyte transmembrane adaptor protein is LAT (linker for activation of T cells), which is critically involved in T cell receptor signalling, but also plays important roles in signal initiation by several other immunologically important receptors. Here we review recent progress in the elucidation of several aspects of this protein, e.g. the controversy concerning the importance of LAT being present in membrane rafts, the involvement in signalling through a number of receptors other than the T cell receptor and the puzzling phenotype of some LAT mutants.