For many years, only two bone resorption markers, urinary hydroxyproline and serum tartrate-resistant acid phosphatase, have been clinically employed. However, these two markers have lacked the sensitivity and specificity to bone resorption. In the late 1980s, pyridinoline (Pyr) and deoxypyridinoline (Dpyr) which are mature crosslinks inter-molecular of collagen, were proposed as bone resorption markers. After newly synthesized collagen is incorporated into bone matrix, Pyr, Dpyr are synthesized by the post-translational modification of collagen molecules. Pyridinoline crosslinks occur in type I collagen at two sites, the carboxy- and amino- terminal telopeptides to helix. At the time of bone resorption, Pyr, Dpyr and amino-, and carboxy- terminal crosslinked telopeptides of type I collagen are excreted into the circulation through collagen degradation. Because the crosslinks and crosslinked telopeptides do not exist at the time of synthesis of collagen, the assay for the crosslinks and telopeptides are specific to bone resorption. Recently, novel assays for measuring crosslinked telopeptides have been developed. Amino-terminal pyridinolines crosslinked telopeptides (NTx) and carboxy-terminal telopeptide of type I collagen (CrossLaps) is assayed in urine, and carboxy-terminal pyridinolines crosslinked telopeptides (ICTP) is assayed in serum. Because those markers are specific to bone resorption, they are expected to reflect the minimum amount of bone resorption in postmenopausal women and osteoporosis. We reviewed the background of Pyr and cross-linked telopeptides markers and compared the performance of these bone resorption markers in the bone turnover in postmenopause and established osteoporosis.