This article reviews the progress made in the studies of copper in silicon over the last several years and puts forward a comprehensive model of the behavior of copper in silicon. Technical aspects of this model are discussed in detail. It is shown that many important aspects of the behavior of copper in silicon are not shared with the other 3d transition metals. The positive charge state of interstitial copper makes its defect reactions Fermi-level-dependent, and results in a noticeable difference in the out-diffusion and precipitation behavior of copper in n-Si and p-Si. The extremely high diffusivity of copper in silicon, which is a consequence of the small ionic radius of copper and its relatively weak interaction with the silicon lattice, makes it highly mobile at room temperature and impacts the stability of copper complexes. Large lattice strains and electrostatic effects in p-Si make the formation of copper-silicide precipitates in the bulk energetically unfavorable, unless the chemical driving force for precipitation is high enough to overcome the nucleation and precipitation barrier. Literature data on the effect of copper on minority carrier lifetime and device yield are analyzed using our improved understanding of the physics of copper in silicon. Finally, the impact of the physics of copper in silicon on the development and characterization of copper diffusion barriers is discussed.