When developing, maintaining, or evolving a system, developers need different types of knowledge (e.g., domain, processes, architecture). They may have memorized (but potentially not documented) the knowledge they perceive important, while they need to recover knowledge that they could not memorize. Previous research has focused on knowledge recovery, but not on what knowledge developers’ consider important to memorize or document. We address this gap by reporting a survey among 37 participants in which we investigated developers’ perspectives on different types of knowledge. Our results indicate that the developers consider certain types of knowledge more important than others, particularly with respect to memorizing them—while all of them should be documented, using specific means. Such insights help researchers and practitioners understand developers’ knowledge and documentation needs within processes, thereby guiding practices and new techniques.