Soy is characterised by a higher content of specific proteins and isoflavones. The question is to know if incorporation of soy foods in the diet may have a favourable effect, or not, on the risk of diabetes, on glycaemic index and insulinemic response, and which are the components involved in these effects. This literature review analyses the epidemiological, clinical and experimental data for that question. Studies are in favour of a beneficial effect of soy on glycaemia, type 2 diabetes risk, and probably on complications of this disease. Its place in the diet should be examined.