The integrity of the skeleton is maintained by two tightly coupled processes: bone resorption and bone formation. Osteoclasts are responsible for bone degradation, osteoblasts for synthesis of the bone matrix. Mature bone consists of a mineral phase (hydroxyapatite crystals) and the organic bone matrix (predominantly type I collagen). In metabolic and malignant bone disease, the balance between bone formation and bone resorption processes is disturbed. Biochemical markers of bone metabolism fall in two categories: enzymes reflecting the activity of osteoblasts (e.g., bone alkaline phosphatase), and osteoclasts (e. g., tartrate-resistant acid phosphatase), and markers of the organic bone matrix. Osteocalcin is established as a bone formation parameter, whereas pyridinium cross-links (a specific breakdown product of type I collagen) can be recommended as a bone resorption parameter. The urinary calcium excretion rate cannot be considered as a specific marker for bone resorption, as it reflects overall mineralization/demineralization rather than bone matrix resorption. Thus, assessment of pyridinium cross-links and urinary calcium excretion are reflecting different aspects of bone turnover. The different biochemical markers of bone metabolism are reviewed in this report, and their applications in states of altered bone metabolism (e.g., malignant osteolysis) are discussed.