The principle of complementarity: chemical versus biological space.
Chemical genomics aims to systematically explore the interactions between small molecules and biological systems. These efforts aim to annotate genomes using the language of chemistry, and to provide information-rich profiles of chemical and biological systems. Here, I describe recent conceptual and experimental advances toward the goal of mapping multidimensional chemical and biological descriptor spaces. In doing so, I will focus on the complementary nature of these efforts, the importance of recognizing the distinction between computed versus observed descriptors, and highlight recent 'landmark' examples of small molecules discovered using phenotypic screens. Future computation and experimental advances will be needed to fully realize the goals of chemical genomics. For those willing to consider both local and global properties of chemical and biological space, and to venture into uncharted territory, there promises to be new vistas and principles to be discovered.