<p>Are students allowed to sit in on lectures for courses they are not enrolled in?</p>
<p>I don't know, but if it's a large lecture you can just show up, because everybody will assume that they just didn't see you before. </p>
<p>I don't think there's a hard and fast rule against it - I'm sure for smaller lectures you would just have to ask the teacher's permission before class if you had a legit reason (you're thinking of taking the course next quarter, for example).</p>
<p>If you audit the course, you are officially enrolled, you just won't get credit. Not auditing the course and showing up in lecture is not a big deal as far as large classes is concern where attendance is not mandatory.</p>
<p>Some professors do not allow auditors. I am thinking specifically of Allen Sanderson for the introductory econ courses. He says specifically that everybody in the room must be enrolled and taking the course for a grade (quality or P/F). He has excellent facial recognition, so I wouldn't try sneaking in there.</p>