Causes renal toxicity Most powerful bactericidal antibiotics Lack of systemic activity after oral administration They frequently consist of several structurally similar but chemically distinct entities isolated from single source MOST ARE CYCLIC, few exceptions (gramicidins) Frequently contain D-amino acids and/or 'unnatural' AA not found in higher plants or animals Contain non-AA moieties, such as heterocycles, fatty aids, sugars, etc