Objective: The aim of this guideline was to formulate practice guidelines for the diagnosis and treatment of Paget's disease of the bone., Participants: The guideline was developed by an Endocrine Society-appointed Task Force of experts, a methodologist, and a medical writer., Evidence: This evidence-based guideline was developed using the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) system to describe both the strength of recommendations and the quality of evidence., Consensus Process: One group meeting, several conference calls, and e-mail communications enabled consensus. Committees and members of The Endocrine Society and the European Society of Endocrinology reviewed and commented on preliminary drafts of these guidelines. Two systematic reviews were conducted to summarize supporting evidence., Conclusions: We recommend that plain radiographs be obtained of the pertinent regions of the skeleton in patients with suspected Paget's disease. If the diagnosis is confirmed, we suggest that a radionucleotide bone scan be done to determine the extent of the disease. After diagnosis of Paget's disease, we recommend measurement of serum total alkaline phosphatase or, when warranted, a more specific marker of bone formation or bone resorption to assess the response to treatment or evolution of the disease in untreated patients. We suggest treatment with a bisphosphonate for most patients with active Paget's disease who are at risk for future complications. We suggest a single 5-mg dose of iv zoledronate as the treatment of choice in patients who have no contraindication. In patients with monostotic disease who have a normal serum total alkaline phosphatase, we suggest that a specific marker of bone formation and bone resorption be measured, although these may still be normal. Serial radionuclide bone scans may determine the response to treatment if the markers are normal. We suggest that bisphosphonate treatment may be effective in preventing or slowing the progress of hearing loss and osteoarthritis in joints adjacent to Paget's disease and may reverse paraplegia associated with spinal Paget's disease. We suggest treatment with a bisphosphonate before surgery on pagetic bone.