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Anodic oxidation, or anodising, is the electrochemical growth of an oxide film on a metal made the anode in an electrolyte, where the oxide thickens because the electric field drives ions through the film already formed. On aluminium and titanium the process gives an adherent, ordered oxide rather than a loose corrosion product, and it serves both to protect the metal and to produce a functional oxide layer. Which of the two outcomes is obtained depends on the balance between oxide growth and oxide dissolution in the electrolyte: in a fluoride-containing solution the dissolution is fast enough that pores nucleate and, under the right voltage and pH, self-organise into a regular array of nanotubes whose diameter and length are set by the anodising conditions ^[1]. The self-ordering and the range of morphologies accessible have been reviewed, together with the behaviour of the resulting arrays in photocatalysis and electrocatalysis ^[2]. The electrolyte composition is the single most decisive variable, since it fixes both the dissolution rate and the species incorporated into the oxide ^[3]. Conducting and transparent supports allow the film to be taken directly as an electrode ^[4], and nanoporous layers grown on bulk metal are examined for their photocatalytic activity ^[5]. Cracking and the limited thermal stability of the amorphous oxide are the practical constraints.