Molecular imaging represents a new term for a long-standing quest to image cellular and molecular processes in vivo. The development of a successful molecular imaging approach starts with a well-defined diagnostic question best answered using in vivo imaging. A selective target for a particular disease state is then identified and a biocompatible probe selective for that target is developed. Many of the challenges of finding selective disease targets and probes that bind selectively to those targets in vivo are evident in the 25-year history of molecular imaging in Alzheimer's disease. This article provides a brief overview of molecular imaging in Alzheimer's disease and its potential for early diagnosis and treatment development.