This chapter investigates the theoretical background of environmental and ecological factors that can be used to inform the design of ecoregional green roofs. Ecoregions are defined by major or minor delineations of plant communities and their interactions with other resident or transient organisms. By observing and learning how native vegetation adapts and thrives in its natural settings, green roof researchers, educators, and designers can learn how to make decisions about the resourceful use of native vegetation on green roofs. This chapter discusses how green roofs must respond to environmental factors such as heat stress, drought, and varied slope and soil conditions, and how these factors can inform the design of green roofs with native vegetation. The chapter ends with a discussion regarding how ecoregions are defined in this book and are employed in the case studies in Part II of this book.