Multiple myeloma must be distinguished from monoclonal gammopathy of undetermined significance (MGUS), smoldering multiple myeloma (SMM), and several other closely related plasma-cell disorders. In recent years, new systems have emerged for diagnosis, staging, and risk stratification of myeloma. The criteria recommended are primarily derived from the International Myeloma Working Group, with certain updates and clarifications. The International Staging System (ISS) is the standard for staging of myeloma. However, for therapeutic purposes, a risk-stratification model is used to define high-risk patients who can benefit from novel therapeutic strategies. The International Myeloma Working Group uniform response criteria have been developed as the standard for response assessment in current and future clinical trials. The criteria incorporate the category 'very good partial response' (VGPR) which, given its importance in predicting outcome following therapy, should be reported in all trials; such reporting will also enable better comparisons between therapies.