Nanoscale science and technology (NST) is an important new field in modern science. In the current study, we seek to answer the question: "What are the essential concepts of NST that should be taught in high school"? A 3-round Delphi study methodology was applied based on 2 communities of experts in nanotechnology research and science education. Eight essential concepts in NST were identified. Each concept is accompanied by its explanation, definition, importance and includes subcategories that compose it. Three concepts emerged in the Delphi study, which were not identified before: functionality, classification of nanomaterials, and the making of nanotechnology. Differences between the concepts suggested by the 2 communities of experts were found. The results of this study serve as a tool to examine different nanotechnology programs that were reported thus far and to make recommendations for designing a NST program for high school students that includes the essential concepts.