MXenes are a large family of two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides. The MXene family has expanded since their original discovery in 2011, and has grown larger with the discovery of ordered double transition-metal (DTM) MXenes. These DTM MXenes differ from their counterpart mono-transition-metal (mono-M) MXenes, where two transition metals can occupy the metal sites. Ordered DTM MXenes are comprised of transition metals in either an in-plane or out-of-plane ordered structure. Additionally, some DTM MXenes are in the form of random solid solutions, which are defined by two randomly distributed transition metals throughout the 2D structure. Their different structures and array of transition-metal pairs provide the ability to tune DTM MXenes for specific optical, magnetic, electrochemical, thermoelectric, catalytic, or mechanical behavior. This degree of control over their composition and structure is unique in the field of 2D materials and offers a new avenue for application-driven design of functional nanomaterials. In this article, we review the synthesis, structure, and properties of DTM MXenes and provide an outlook for future research in this field.