Nanotechnology will have great impact on how cancer is diagnosed and treated in the future. New technologies to detect and image cancerous changes and materials that enable new methods of cancer treatment will radically alter patient outcomes. The National Cancer Institute (NCI) Alliance for Nanotechnology in Cancer sponsors research in cancer prevention, diagnosis, and therapy and promotes translation of basic science discoveries into clinical practice. The Fourth Annual NCI Alliance Principal Investigator Meeting was held in Manhattan Beach, California October 20-22, 2009. Presented here are highlights from the research presentations at the meeting, in the areas of in vitro diagnostics, targeted delivery of anticancer and contrast enhancement agents, and nanotherapeutics and therapeutic monitoring.