Advance directives are important to preserve the autonomy of patients for future situations in which they are uncapable of expressing their will. They are considered helpful by many healthcare professionals in their professional practice. However, their knowledge on these documents is not well known. Misconceptions can adversely affect decisions at the end of life. This study examines healthcare professionals' knowledge of advance directives and relevant correlates. In 2021 healthcare professionals from various professions and institutions in Wuerzburg were surveyed using a standardized questionnaire on previous experiences with, advice on and use of advance directives, as well as an objective knowledge test containing 30 questions. Apart from the descriptive analysis of single questions out of the knowledge test, various parameters were screened for their influence on knowledge level. 363 healthcare professionals from different care settings participated in the study, including physicians, social workers, nurses and emergency services personnel. 77.5% work in patient care, of which 39.8% make decisions based on living wills daily to several times a month. High rates of incorrect answers in the knowledge test show lack in knowledge about decisions concerning patients who are unable to give consent; an average of 18 out of 30 points was achieved. Physicians, male healthcare professionals and respondents with more personal experience regarding advance directives had significantly better results in the knowledge test. Healthcare professionals have ethically and practically relevant knowledge deficits and a high need for further training on advance directives. Advance directives play an important role in maintaining patient autonomy and should receive more attention in training and further education equally involving non-medical professional groups.