Teeth move through alveolar bone through naturally occurring drift or in response to orthodontic forces. Orthodontists want to optimize this movement while reducing potential risk factors. Orthodontic researchers have taken on this clinical challenge by uncovering the biological phenomena associated with tooth movement. There is a general consensus that the major biological events that permit orthodontic tooth movement are bone resorption to remove alveolar bone in the path of movement followed by bone formation to maintain the integrity of alveolar bone. The rates of bone resorption and tooth movement are directly proportional, while the rate of bone formation determines treatment success. In broader terms, orthodontic tooth movement can be divided into two phases: bone resorption occursduring the catabolic phase, and bone formation occurs during the anabolic phase. While we generally agree on the overall cellular and histological events necessary for orthodontic movement, the detailed mechanisms mediating these events are not completely understood. How do orthodontic forces activate bone resorption and formation? Do orthodontic forces directly or indirectly induce tooth movement? Does the periodontal ligament (PDL) influence the rate of tooth movement? Toaddress these questions, we will begin by examining how bone cells function.