Understanding how systems form and behave is a fundamental pursuit at the frontier of many scientific disciplines covering both the natural and human worlds. Limitations are increasingly evident in the traditional reductionist approach to science: that is, breaking systems apart and studying the component parts in order to figure out how the system works as a whole. Much of what we know about nature is a product of this approach, but current system oriented management and knowledge gaps demand more. To truly understand ecological systems, or others such as economic and social systems, will require a different kind of science and a different way of thinking. Ecosystem science, and the larger specialty of complexity, are just beginning to emerge as prominent pursuits for this generation of researchers. Emergent properties, phase transitions, criticality, stable states, nonlinear relations, and other related concepts are developing from scattered advancements in a variety of fields. Applying such concepts to environmental systems is just beginning through pioneering research of a relatively few scientists. Mark Bain has put a particular focus on his systems research approach: integrating hydrology and aquatic ecology as a major organizing dimension in aquatic ecosystems. This pursuit, sometimes called hydroecology or biohydrology, lacks principles or methods but it is increasingly seen as a promising area for advancing understanding and the development of management approaches.