Abstract The mechanism of electroless nickel plating (Ni-P plating) is first described. Nickel ions are discharged by the electrons of the metal to be plated (steel, aluminium, etc.). Because of attack by the acid plating solution, the nickel nuclei are formed on the surface. Once formed, catalytic reduction of nickel ions by NaH 2 PO 2 takes place at these nuclei. The structure of the deposit and its variation by heat treatment is then discussed. Elementary layers with a thickness of ca. 0.3 μm together with a super-imposed lamellar structure as a function of phosphorus concentration and internal stresses are found in the as-plated state. Diffusion processes in the deposit and between the deposit and the substrate influence the structure, hardness, toughness and other properties of the NiP alloy (corrosion, abrasion, erosion, cavitation resistance). Some of these correlations are described.