Recent studies on structures and functions of sweetness-inhibiting substances (gymnemic acid, ziziphin, and gurmarin); sweet proteins (monellin, thaumatin and mabinlin); and taste-modifying proteins (miraculin and curculin) were reviewed. Several gymnemic acid homologues and gurmarin were purified from the leaves of Gymnema sylvestre and their structures were determined. Ziziphin was also purified from leaves of Ziziphus jujuba. Gymnemic acid and ziziphin are glycoside of triterpenes that suppress sweetness in human, while gurmarin is a peptide having antisweet activity in rat. Mabinlin is a heat-stable sweet protein. The whole amino acid sequence and the position of disulfide bridges of mabinlin were determined. Miraculin has the unusual property of modifying a sour taste into a sweet taste. Curculin elicits a sweet taste. In addition, water and sour substance elicit a sweet taste after curculin. Their amino acid sequences and subunit structures were determined. These proteins are expected to be used as low-calorie sweeteners.