BACKGROUND: Adequate vitamin D is essential for maintaining optimal bone health, preventing and treating of osteoporosis. However, in recent years, large clinical trials and meta-analyses on the efficacy of vitamin D supplementation to prevent fractures in populations at different risks have been equivocal. The optimal level of 25-hydroxyvitamin D (25[OH]D) remains controversial. Recommendations vary between societies. The lack of standardized assays also poses a challenge in interpreting available research data. METHODS: We systematically searched for articles in MEDLINE database through PubMed, which included meta-analysis, systematic reviews of randomized controlled trials (RCTs) and observational studies that assessed measurement, diagnosis and treatment about vitamin D deficiency. The experts evaluated the available literature, graded references according to the type of study and described the strength recommendations. RESULTS: This expert consensus is based on the review of relevant clinical evidence and provides nine key recommendations on vitamin D deficiency in populations at different risks, especially in patients with osteoporosis. Supporting information is provided in the subsequent appendix box. CONCLUSIONS: This expert consensus is a practical tool for endocrinologists, general physicians for the diagnosis, assessment, and treatment of populations at different risks of vitamin D deficiency, especially in patients with osteoporosis. Clinicians should be aware of the evidence but make individualized decisions based on specific patients or situation.