Monoclonal gammopathies range from benign conditions to severe malignancies. A summary is given in this review. Overall, the prevalence is high; monoclonal gammopathies (MGUS) occur in > 3% of persons above 50 years of age. Approximately 400 new cases of multiple myeloma and 80 new cases of amyloid light-chain (AL) amyloidosis are diagnosed yearly in Denmark. MGUS is most often asymptomatic, but M-protein associated syndromes exist and should be considered when finding M-protein. Serum free light kappa and lambda chain analysis, CT, PET/CT and whole-body MRI have revolutionised diagnostics and monitoring of monoclonal gammopathies. New treatment modalities have improved outcome in multiple myeloma and AL amyloidosis.