A number of thought experiments are cited, some well-known, some not. These illustrate the power of thought experiments. Other examples are given that show some of the dangers. As well as examples from the science, some examples of visual reasoning from mathematics are also presented, again with an eye to illustrating their promise and perils. Morals are drawn, perhaps the most important being that we still have much to learn about them, just as we once learned about the promise and perils of microscopes in their early days. They are a tool that can mislead, but they can also open up a whole new world.