Bone metastases are the most common malignant bone tumors. Skeletal involvement varies in different human malignancies, with breast cancer being the leading cause for bone metastases in women and prostate cancer in men, followed by lung cancer. Bone is the first site of relapse in 50% of patients with breast cancer and in 80% of patients with prostate cancer. Various morphologic and functional imaging modalities are being used for assessment of malignant bone involvement. The purpose of imaging is to identify malignant bone involvement as early as possible, to determine the extent of the disease, to evaluate the presence of complications, to monitor response to therapy, and occasionally to guide biopsy. Hybrid techniques composed of either SPECT or PET installed in the same gantry with CT are commercially available. 18F-FDG, the most commonly used PET tracer, is a glucose analog that directly identifies tumor tissue characterized by high metabolic activity. It is, therefore, sensitive for detection of early bone marrow involvement prior to any identifiable bone changes and lytic-type bone metastases. 18F-fluoride is a bone-seeking PET tracer. Its uptake depends on regional blood flow and osteoblastic activity; the better spatial resolution of PET and the favorable pharmacokinetic characteristics of 18F-fluoride make 18F-fluoride-PET a highly sensitive modality for detecting both lytic and blastic lesions. Novel hybrid systems composed of PET and CT allow for acquisition of both modalities at the same clinical setting and the generation of fused functional-anatomical images.