Btk is a cytoplasmic tyrosine kinase, which is mainly involved in B cell receptor signalling. Gene targeting experiments revealed that Btk is important for B cell development and function. However, Btk is not only expressed in B cells, but also in most other haematopoietic lineages except for T cells and plasma cells. Recently we found that Btk is involved in Toll-like receptor signalling. Toll-like receptors play an important role in innate immunity. They are highly expressed on mast cells, macrophages and dendritic cells, which are essential for the recognition and consequently for the elimination of microbial pathogens. Therefore Btk might play an important role for the function of immunocompetent cells of innate as well as adaptive immunity.