A reachability question is a search across feasible paths through a program for target statements matching search criteria. In three separate studies, we found that reachability questions are common and often time consuming to answer. In the first study, we observed 13 developers in the lab and found that half of the bugs developers inserted were associated with reachability questions. In the second study, 460 professional software developers reported asking questions that may be answered using reachability questions more than 9 times a day, and 82% rated one or more as at least somewhat hard to answer. In the third study, we observed 17 developers in the field and found that 9 of the 10 longest activities were associated with reachability questions. These findings suggest that answering reachability questions is an important source of difficulty understanding large, complex codebases.