Summary1. The available methods for histochemical analysis of lipoids having been reviewed, the following are regarded as valid for the substances or groups indicated: Sudan black: all liquid or grease‐like lipoids.Nile blue sulphate: ‘acidic’ lipoids (lipines, chromolipoids and free fatty acids) as against ‘neutral’ ones (hydrocarbons, triglycerides, higher alcohols, waxes, steroids, carotenoids, etc.).Fluorescence (without the addition of sulphuric acid): a rapidly fading green is due to vitamin A.Presence of spherocrystals: lipines, cholesteryl esters (and perhaps other sterol esters as well).Osmium tetroxide, primary blackening only: for active unsaturated bonds, most usually ethylenic linkages in fatty acid radicles and probably in carotenoids, but also for aldehyde groups (in plasmal), and for various other strong reducers which are not lipoid.Acid haematein test: phospholipines (possibly acetalphosphatides as well).Feulgen's plasmal test (not the Feulgen‐Verne test): acetalphosphatides by means of the aldehydes liberated from them.Schultz's method I (modification of Liebermann's reaction): cholesteryl group. Schultz's method II (modification of Lifschiitz's reaction):withouta previous oxidation, for certain ill‐defined oxidation products of sterols,witha previous oxidation (by light and air, or by ferric compounds) for the cholesteryl group.Antimony trichloride, and glycerol dichlorhydrin: atransientblue or blue‐green is specific for carotenoids.2. Some of the above tests are subject to interference by certain rare substances; the plasmal test and Schultz's method I may possibly give positive results with certain toad poisons.3. The following tests are rejected as definitely invalid, insufficiently investigated, or superseded by more satisfactory techniques:Fischler's technique for fatty acids;Casanova's reaction;Ciaccio's methods I and II for ‘lipoids’The Smith‐Dietrich method for phospholipines;Romieu's method for lecithin;The Feulgen‐Verne technique for plasmal;Bennett's