This review summarises the work-up of patients with monoclonal gammopathy of undetermined significance (MGUS). In persons above 70 years of age, around 5% have MGUS, a premalignant state with a monoclonal plasma immunoglobulin or light chain (M protein) in blood and/or urine. Continuous follow-up is recommended due to a risk of malignant progression of around 1% per year. Immunoglobulin M MGUS primarily progresses to Waldenström's macroglobulinaemia, whereas non-immunoglobulin M MGUS typically progresses to multiple myeloma or amyloid light-chain amyloidosis. Treatment is unnecessary unless in rare cases of severe non-malignant complications. Screening is not advised.