Computation must exist in the physical world. Security designs that require secrets must hide and use them in the real world. Unfortunately, the real world offers more paths to secret storage and more observable computational artifacts than these security designs anticipate. Careful integration of physical defenses and security architecture can sometimes succeed against the adversary class designers consider. However, in the long term, we hope for either a quantum leap in physically defensible technology-or a significant reduction in the properties that designs force us to assume about our computers.