Human adenoviruses belong to the Adenoviridae family and they are divided into seven species, including 56 types. Adenoviruses are common opportunistic pathogens that are rarely associated with clinical symptoms in immunocompetent patients. However, they are emerging pathogens causing morbidity and mortality in recipients of hematopoietic stem cell and solid organ transplants, HIV infected patients and patients with primary immune deficiencies. Clinical presentation ranges from asymptomatic viraemia to respiratory and gastrointestinal disease, haemorrhagic cystitis and severe disseminated illness. There is currently no formally approved therapy for the treatment of adenovirus infections. This article presents current knowledge about adenoviruses, their pathogenicity and information about available methods to diagnose and treat adenoviral infections.