This lecture will address the following aspects in orthodontic tooth movement based on a series of studies: what is a good animal model for setting up a research? What is the optimal force for orthodontic tooth movement? Is the efficiency of tooth movement affected by age? How are the profile changes of osteoclasts and periodontal vasculature? What are the truth and myth in root resorption? Can we manipulate the key cells in bone remodeling for clinical application? Finally the potential of GCF as a diagnostic tool in orthodontics is explored and discussed.