IMPORTANCE Using personal protective equipment (PPE) and transmission-based precautions are primary strategies for reducing the transmission of infectious agents. OBJECTIVE To identify and characterize failures in transmission-based precautions, including PPE use, by health care personnel that could result in self-contamination or transmission during routine, everyday hospital care. DESIGN, SETTING, AND PARTICIPANTS This qualitative study involved direct observation inside and outside patient rooms on clinical units from March 1, 2016, to November 30, 2016. Observations occurred in the medical and/or surgical units and intensive care units at an academic medical center and a Veterans Affairs hospital, as well as the emergency department of the university hospital. Trained observers recorded extensive field notes while personnel provided care for patients in precautions for a pathogen transmitted through contact (eg, Clostridium difficile, methicillin-resistant Staphylococcus aureus) or respiratory droplet (eg, influenza). Specific occurrences involving potential personnel self-contamination were identified through a directed content analysis. These occurrences were further categorized, using a human factors model of human error, as active failures, such as violations, mistakes, or slips. MAIN OUTCOMES AND MEASURES Number and type of failures involving use of transmission-based precautions. RESULTS In total, 325 room observations were conducted at 2 sites. At site 1, a total of 280 observations were completed (196 in medical/surgical units, 64 in intensive care units, and 20 in emergency departments). At site 2, there were 45 observations (36 in medical/surgical units and 9 in the intensive care unit). Of the total observations, 259 (79.7%) occurred outside and 66 (20.3%) inside the room. Two hundred eighty-three failures were observed, including 102 violations (deviations from safe operating practices or procedures), 144 process or procedural mistakes (failures of intention), and 37 slips (failures of execution). Violations involved entering rooms without some or all recommended PPE. Mistakes were frequently observed during PPE removal and encounters with challenging logistical situations, such as badge-enforced computer logins. Slips included touching one's face or clean areas with contaminated gloves or gowns. Each of these active failures has a substantial likelihood of resulting in self-contamination. The circumstances surrounding failures in precaution practices, however, varied not only across but within the different failure types. CONCLUSIONS AND RELEVANCE Active failures in PPE use and transmission-based precautions, potentially leading to self-contamination, were commonly observed. The factors that contributed to these failures varied widely, suggesting the need for a range of strategies to reduce potential transmission risk during routine hospital care.