Abstract The part played by silicon compounds in terrestrial life can be discussed under three main headings. First is the important skeletal role which silica fulfils in organisms such as diatoms. Secondly is the reduction of silicate which can be performed by various micro-organisms and the question whether this can be regarded as a specific participation in metabolism. Thirdly is the toxic effect of silicon compounds in higher vertebrates, including man, which is responsible for the disease silicosis in miners and for induction of certain types of malignant tumours. From these observations it is clear that silicon compounds play an interesting, but relatively minor and incidental role, in terrestrial life. The question then arises whether this was a chance happening in the origin of life on our planet, or whether there are any properties of silicon which disqualify it from more direct participation in metabolism, so that it could not substitute for some other central element such as carbon in extraterrestrial life forms. Some general properties of silicon compounds that bear on this problem will be discussed.