The non-invasive assessment of bone turnover has received increasing attention over the past few years because of the need for sensitive markers in the clinical investigation of osteoporosis. Markers of bone formation include serum total and bone-specific alkaline phosphatase, serum osteocalcin, and measurement of serum type I collagen extension peptides. Assessment of bone resorption can be achieved with measurement of urinary hydroxyproline, urinary excretion of the pyridinium crosslinks (Pyr and D-Pyr), and by measurement of plasma TRAP activity. For the screening of bone turnover in women at the menopause, and for the assessment of the level of bone turnover in elderly women with vertebral osteoporosis, serum osteocalcin and urinary Pyr and D-Pyr appear to be the most sensitive markers so far. Programmes combining bone mass measurement and assessment of bone turnover in women at the time of the menopause have been developed in an attempt to improve the assessment of the risk for osteoporosis. Efforts are being made to develop more convenient assays and to identify other markers of bone turnover. In the future a battery of various specific markers is likely to improve the assessment of the complex aspects of bone metabolism, especially in osteoporosis.