Multimorbidity is often defined as two or more long-term conditions, the definition may, however, vary. This review summarises various definitions of multimorbidity. The prevalence of multimorbidity in Denmark is between 7% and 29% depending on data sources and definition and is increasing with age; nonetheless most patients with multimorbidity are of working age. Several multimorbidity indices have been developed for research purposes, but with no clinical consensus. The concept of complex multimorbidity adds psychosocial context and health-care patterns to better describe the group of patients with multimorbidity having the highest needs.