In the processing of photographic films, plates or papers, the photographic developer (or just developer) is a chemical that makes the latent image on the film or print visible. It does this by reducing the silver halides that have been exposed to light to elemental silver in the gelatine matrix. As a generalisation, the longer a developer is allowed to work, the greater the degree of reduction of the silver halide crystals to silver and therefore the darker the image.

At the start of photography a wide range of developing agents were experimented with including human urine. Very soon more successful agents came into use such as salts of iron such as iron oxalate, iron sulfate and iron lactate as in the ferrotype, wet plate and ambrotype processes.

For black-and-white photography, the developer is typically a mixture of metol (monomethyl-p-aminophenol hemisulfate), phenidone (1-phenyl-3-pyrazolidinone) or dimezone (4,4-dimethyl-1-phenylpyrazolidin-3-one) and hydroquinone (benzene-1,4-diol). These are made up in aqueous solution with a suitable alkaline agent such as sodium carbonate, borax, or sodium hydroxide to create the appropriately high pH and with sodium sulfite to delay oxidation of the developing agents by atmospheric oxygen. Hydroquinone is superadditive with metol, meaning that it acts to "recharge" the metol after it has been oxidised in the process of reducing silver in the emulsion. Sulfite in a developer not only acts to prevent aerial oxidation of the developing agents in solution, it also facilitates the regeneration of metol by hydroquinone (reducing compensation and adjacency effects) and in high enough concentrations acts as a silver halide solvent.