The role of transition metal complexes as catalysts for homogeneous hydrogenation reactions has been extensively investigated during the last few decades. This paper reviews recently disclosed work by a number of research laboratories including ours, which shows that the study of the kinetics of such processes is a particularly important tool which, if used in combination with spectroscopic techniques, isotope labeling, isolation of stable analogues and theoretical calculations, allows the elucidation of the mechanisms operating in a variety of homogeneous catalytic hydrogenation reactions.