Numerous experimental and clinical observations suggest that overall changes in bone resorption during menopause or treatment with hormone replacement therapy (HRT) are combined effects of changes in osteoclast number and function. Moreover, due to a coupling between osteoclastic bone resorption and osteoblastic bone formation, pronounced alteration of osteoclast number will eventually lead to alteration of osteoblastic bone formation. Fragments of type I collagen, such as the C- and N-terminal telopeptides of collagen type I (CTX and NTX, respectively), are generated during bone resorption and hence can be used as surrogate markers of osteoclast function. Circulating levels of different enzymes in the serum, such as TRAP 5b and cathepsin K are proportional to the number of osteoclasts, and hence can be used as surrogate markers of osteoclast number. Since antiresorptive effects can be obtained in different ways, we felt it was timely to discuss the different scenarios, highlight differences specific to different pharmacological interventions with different mechanisms of action, and discuss how these bone markers can assist us in a deeper analysis of the pharmacodynamics and safety profile of existing and upcoming drug candidates.