Toll-like receptors (TLRs) are a growing family of molecules involved in innate immunity. Accumulating evidence suggests that TLR molecules are involved in signalling receptor complexes which recognise components of Gram-positive and Gram-negative bacteria and mycobacteria. Differential expression and regulation as well as distinct though overlapping ligand recognition patterns may underlie the existence of a vast TLR family. Apparent structural and functional redundancy may render certain outputs of the TLR family robust.