Each chapter in this volume describes in detail the application of one or a group of photosensitive molecules to biological research. In this chapter, we take up general prefatory questions: how to determine which molecules are appropriate to use, and what type of compound delivery and light-targeting apparatus for photoactivation is likely to give satisfactory spatial and temporal performance. We enumerate the advantages and disadvantages of currently available "caged" and genetically encoded photosensitive molecules. We also compare current mature and emerging technologies for patterned light delivery, referring as much as possible to broadly applicable general principles. Our goal is to provide a comprehensive overview with signposts to more detailed treatments.