This paper gives an overview of the problems related to metal speciation measurements and interpretation, as well as the criteria which should be used for the development of in situ techniques. Three promising methods are examined in greater detail: diffusion gradients in thin films (DGT), integrated diffusion in gel and voltammetry and the supported liquid membrane (SLM) preconcentration technique. The present state of the art in this field is discussed with suggestions for future research directions.