This article gives a critical review of the recent progress in the understanding of the physical properties of defects on Si(001). Mainly two classes of defects will be considered: point defects, including adatoms, ad-dimers, atom vacancies and dimer vacancies; and line defects, including steps, dimer strings, antiphase boundaries and vacancy lines. The focus will be on the kinetic and dynamic properties of the defects. Also discussed are their mutual interactions. A combination of extensive theoretical and experimental studies has provided clear answers to several fundamental questions about defect energetics and dynamics, and in some other cases has resulted in plausible solutions. In surveying some open or controversial issues, likely future directions will be discussed as well.