Application prospects in automotive industry have led to extensive studies on 6xxx alloys in recent years. These alloys often contain Cu in varying amounts. This leads to the formation of the quaternary Al-Mg-Si-Cu family of alloys that may exist either as a 6xxx or a 2xxx alloy. These alloys have distinctive properties in part due to the occurrence of a phase, designated as Q, which is stable only as a quaternary compound with variously reported stoichiometry. In this paper we first review the equilibrium phase field of various Al-Mg-Si-Cu alloys, noting the many important commercial alloys that contain the Q phase as an equilibrium one. We review the metallographic and crystallographic aspects of the Q phase. One of the important precursors of the Q phase is the Q 0 phase, and its crystallography and microstructural features are presented and discussed. Various other competing metastable phases in both ternary and quaternary systems are presented and an attempt is made to systematize their occurrence with respect to overall alloy composition. The metastable precursor phase Q 0 has the same crystal system and similar composition as Q. Recent literature indicates that a phase with the same crystal system, orientation relations and lath morphology as Q 0 is also found in ternary Al-Mg-Si alloys with Si contents in excess of the balanced composition. This phase is metastable and is replaced by the equilibrium b phase. Finally, all of these alloys have been reported to be strengthened primarily by b 00 phase during artificial aging such as during paint bake ( 180 C). The source of such strengthening in quaternary alloys is critically analyzed in light of conflicting reports on the ubiquity of b 00 phase and studies on the role of the Q 0 and other likely metastable phases.