The various stages in the production of an aluminium electrolytic capacitor are outlined and these are then related to the equivalent circuit so as to establish an understanding of the preparation parameters that affect the performance. Particular reference is made to studies undertaken on the effects of etching prior to anodization, the anodization itself, the electrolyte used and the mechanical construction. In the case of sintered porous tantalum electrolytic capacitors, the parameters are not so well understood but comparisons will be drawn with aluminium capacitors wherever possible.