Teachers of physiology at all postsecondary levels were asked to participate in a survey about the possible sources of students' difficulty in learning physiology. Sixty-three physiology teachers responded to the 18-question survey; 35 of the respondents also responded to a request for written comments about this issue prior to taking the survey. Three categories of possible factors contributing to physiology being hard to learn were defined: 1) the nature of the discipline, 2) the way it is taught, and 3) what students bring to the task of learning physiology. Respondents thought that characteristics of the discipline (it requires causal reasoning, it uses graphs and mathematics, and it is highly integrative) and characteristics of students (they believe that learning and memorizing are the same thing, they cannot or will not attempt to integrate, and they compartmentalize) were significantly more important than any aspect of teaching in making physiology hard to learn. Recommendations are offered in this article to help students deal with the sources of difficulty that were identified.