Hospital-acquired infections (HAIs) or nosocomial infections (Greek: nosokomeion = hospital) are defined in human medicine as 'all clinically apparent infections that were not present or incubating in the patient prior to hospital admission'. They typically occur 48 hours or later after admission, within 3 days of discharge or within 30 days of an operation. HAIs also include infections that occur in hospital staff from pathogens acquired at work. Of particular concern nowadays are HAIs caused by multidrug-resistant bacteria. These are linked to the frequent use of antimicrobials and antiseptics in human hospitals and multidrug-resistant organisms have also emerged in animal hospitals. If an unexpected or unusual series of HAI cases is recognized, they are referred to as outbreaks and require a prompt and rigorous search for a common source to reduce their impact. In humans, nosocomial infections lead to increases in morbidity and mortality and may prolong the length of stay in hospital. The associated financial burden for providers of human healthcare is substantial. The chapter covers Types of infection; Pathogens; Incidence and risk factors; Prevention; and Investigation of occurrences.