Pagets bone disease (PDB) is a focal metabolic disorder that affects 2-3% of the population older than 60 years and is characterized by increased and grossly distorted bone remodeling, bone hypertrophy, and abnormal bone structure. The disease affects one or several bone pieces and its aetiology remains unclear. The primary cell abnormality in PDB could involve the osteoclasts that are markedly increased in number and size, can have 100 nuclei per cell, and contain paramyxoviral-like nuclear and cytoplasmatic inclusions. Thirty years ago the observation of these inclusions suggested the involvement of paramyxovirus in the PDB pathogenesis. Nevertheless, the paramyxoviral theory is a very controversial one. On the other hand, familial aggregation studies indicate that 40% of affected subjects show a PDB familial history and recent advances in our understanding of the disease come from genetic studies. PDB is genetically heterogeneous, with seven loci (PBD 1-7) reported at this time. Most likely, an interaction between environmental and genetic factors is required for patients to develop PDB and could explain the geographic distribution of the disease as well as its peculiar variable phenotypic presentation.