The correction of posterior crossbites is more complex than it appears. To develop an appropriate treatment plan, it is first necessary to determine if: (1) there is a functional jaw shift on closing; (2) the crossbite is unilateral or bilateral; (3) it is dental, skeletal, or a combination of both; and (4) it is related only to the maxilla or both jaws. Once the problem's apparent cause has been defined, it is then necessary to select the appropriate modality of treatment, which, in the maxilla, is: (1) usually a removable acrylic-based appliance with 1 or more transverse screws; or (2) a fixed Hyrax-type or Hass-type appliance. In the mandible, if dental expansion is required, the appliances most frequently used are the lip bumper or a removable acrylic-based appliance with a single screw lingual to the incisors. Once the appliance has been placed, it must be determined when adequate expansion has been achieved and how best to retain it.