Hospital-acquired infections (HAIs) or nosocomial infection (NI) are defined as infections that develop after 48 hours of hospitalization, with no evidence that the infection was present or incubating at the time of admission. These infections occur 48 hours after hospital admission or up to 3 days after discharge or up to 30 days after an operation. HAls are most commonly associated with invasive medical devices or surgical procedures. Lower respiratory tract and bloodstream infections are the most lethal; however, urinary tract infections are the most common. Although quantification is not yet possible, many HAIs can be prevented. Hand hygiene is the single most important measure to reduce infection risk. The patient care environment must undergo extensive daily and post-discharge cleaning so that pathogens will not be passed from one patient to another. Specific HAIs prevention aspects, surgical site infections, ventilator-associated pneumonia and hospital-acquired pneumonia, multidrug-resistant organisms, and Gram-negative MDROs are the topics which are covered throughout this chapter.