Silicon (Si) is the second most abundant element in the Earth's crust and although there has been interest in the biological role of Si since the beginning of the century, it is only in the last three decades that it has been suggested as an essential trace element. Si may have a role in a number of areas of human physiology and metabolism, especially bone and connective tissue formation, but possibly also gene expression and cardiovascular health. Si is rarely found in its elemental form as it readily reacts with atmospheric O2 and water to produce silicates. Physiologically, it exists as an ion in the body. These vary in composition from simple soluble orthosilicic acid and oligomers through to less soluble colloids and gels, and insoluble solid-phase silicates. The latter constitute the vast majority of environmental silicates (for example, soil minerals), although natural waters contain soluble silicates (mostly orthosilicic acid) generally at 1 to 20 mg/l. Orthosilicic acid is the most readily absorbed form of Si and silicate in man. Silicon plays an important role in bone mineralization and formation and is therefore incorporated into a wide variety of medical implants and bone grafts used today. Low bone mass (osteoporosis) is a silent epidemic of the 21st century, which presently in the UK results in over 200,000 fractures annually at a cost of over one billion pounds. Figures are set to increase worldwide. Understanding the factors which affect bone metabolism is thus of primary importance in order to establish preventative measures or treatments for this condition. Nutrition is an important determinant of bone health, but the effects of the individual nutrients and minerals, other than calcium, is little understood. Accumulating evidence over the last 30 years strongly suggest that dietary silicon is beneficial to bone and connective tissue health and we recently reported strong positive associations between dietary Si intake and bone mineral density in US and UK cohorts. The exact biological role(s) of silicon in bone health is still not clear, although a number of possible mechanisms have been suggested, including the synthesis of collagen and/or its stabilization, and matrix mineralization. The field of bioorganosilicon chemistry, which sprung up in the 1970s to exploit the opportunities of silicon for drug design, is currently being developed into a practical and commercial enterprise. Some of the fundamental differences between carbon and silicon can lead to marked alterations in the physicochemical and biological properties of the silicon-containing analogues and the resulting benefits can be exploited in the drug design process. Recent evidence confirms the fundamental involvement of the human immune system in the reaction to implantation of silicone based medical devices. An as yet to be particularized epitope of many complex substances sharing siloxane structures is presented through the MHC-II apparatus with development and retention of T cell memory. This memory can be tested for in practical terms using one or more forms of silica, which links the immuno-histopathology and autoimmune attributes of 'silicosis' with those of 'siliconosis'. The lesions of siliconosis are typical of those for persistent antigens and delayed, cell mediated hypersensitivity. The basic descriptive pathology of the reaction to silicone has been known since soon after introduction of silicones in medical procedures, with the exception of some details related to the more recent discoveries on the role of cytokines in the immunopathic process. The clinical consequences of siliconosis are common and can be severe in some individuals implanted with silicone devices. (PMID: 16277785, 16632368, 17435952, 12821303, 12951816, 9951648).