5 According to Newton's Third Law, for every action, there is an equal and opposite reaction. Orthodontists must acknowledge this law every time they try to move teeth. Simply stated, every desired tooth movement has the potential to simultaneously create an undesired tooth movement. Orthodontic anchorage is defined as the resistance to such undesired tooth movement and can be achieved by more or less predictable methods. For space closure, 3 anchorage situations are traditionally defined by the ratio of incisor retraction to molar protraction. Maximum anchorage describes space closure mostly by incisor retraction and, to a lesser degree, by molar protraction (ratio of 2:1). Moderate anchorage comes with equal parts of incisor retraction and molar protraction (ratio of 1:1), whereas minimal anchorage allows spaces to close mostly by molar protraction and less by incisor retraction (ratio of 1:2). Traditionally, this type of anchorage control relies heavily on other appliances that add to the anchorage segment, such as compliance-dependent headgear or the noncompliance-dependent Nance appliance or, for instance, on elastic wear, which again depends on compliance. Although headgear incorporates skeletal structures into the anchorage segment (eg, high-pull headgear uses the occipital bone), elastics incorporate only other teeth into the anchorage segment, and this carries the potential for further side effects in the opposing dental arch. The philosophy behind skeletal anchorage is that if the reactive forces can be absorbed by skeletal structures, tooth movement can be limited to the desired therapeutic movements, and the undesirable reactive side effects can be prevented entirely. Such a situation can nowadays be created by temporarily implanting small devices into the patient's jawbone and using