Flavan-3-ols (sometimes referred to as flavanols) are a class similar to flavonoids that use the 2-phenyl-3,4-dihydro-2H-chromen-3-ol skeleton. These compounds include the catechins and the catechin gallates.

Flavanols (with an "a") are not to be confused with flavonols (with an "o"), a class of flavonoids containing a ketone group.

The single-molecule (monomer) catechin, or isomer epicatechin (see diagram), adds 4 hydroxyls to Flavan-3-ol making building blocks for concatenated polymers proanthocyanidins and (higher order polymers) anthocyanidins.

Flavanols possess two chiral carbons meaning there 4 diastereoisomers for each of them.

Catechins are distinguished from the yellow, ketone-containing flavonoids such as quercitin and rutin, which are called flavonols (spelled flavonols with an o). Early use of the term bioflavonoid was imprecisely applied to include the flavanols, which are distinguished by absence of ketone(s). Catechin monomers, dimers, and trimers (oligomers) are colorless. Higher order polymers, anthocyanidins exhibit deepening reds and become tannins.