Any process that results in a change in state or activity of a cell or an organism (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of a monoamine stimulus. A monoamine is any of a group of molecular messengers that contain one amino group that is connected to an aromatic ring by ethylene group (-CH2-CH2-). Monoamines are derived from the aromatic amino acids phenylalanine, tyrosine, histidine and tryptophan.