This chapter introduces the reader to some basic scientific concepts that are applicable to environmental decision-making. It focuses on natural systems and systems thinking. From an environmental decision-making standpoint, certain aspects of natural systems are considered of crucial importance, one being the issue of biodiversity. The chapter highlights some of the more critical concepts of biodiversity for consideration. Formal definitions of biodiversity differ depending on what scientific discipline is defining the term, and in what context the term is being used. From a genetic standpoint, biodiversity means the diversity of genetic materials being shared between populations of the same species. A more generalized definition of biodiversity tends to account for both the micro-and macro-scales of diversity. The chapter summarizes the major interactions between the biotic and abiotic parts of a natural system. The reason is to identify aspects of these interactions in context, allowing us to see the importance of the entire ecosystem from an environmental decision-making perspective.