The diffusivityD1of silicon self-interstitials plays an important role in the quantitative modeling of many technological processes for microelectronic and photovoltaic silicon devices. Examples are swirl-defect formation during crystal growth, oxygen precipitation, oxidation-enhanced diffusion, oxidation-induced stacking faults, gettering phenomena, and non-equilibrium effects associated with high-concentration phosphorus diffusion. This paper reviews the various attempts to measure or estimateD1, which in spite of all the effort remains a fairly elusive quantity partly because of interactions of self-interstitials with coexisting vacancies and sinks in the material.