Various phenomena associated with phosphorus diffusion in silicon are reviewed and prominent models are critiqued. It is shown that these models are either fundamentally unsound, or are inconsistent with observed phenomena. A consistent model is proposed in which two mechanisms are operating simultaneously, namely, the vacancy mechanism for the slower diffusing component, and the interstitialcy mechanism for the faster diffusing component. It is assumed that phosphorus exists in silicon in both the substitutional and the interstitialcy species, and that both are shallow donors. The conversion between the two species is relatively slow, giving rise to the so‐called kinked concentration profile. Diffusion via a partial interstitialcy mechanism leads to a supersaturation of self‐interstitials.