In this chapter, the disciplinary core idea (DCI) is central to biology and also serves as the glue connecting all four DCIs in the life sciences strand. In essence, it is about the properties of living things and explaining the complexity of life and the living by exploring their components and interactions across multiple biological organization levels. Teaching this DCI entails engaging students with many phenomena they “know” but may not understand the underlying mechanisms of (e.g., why we breathe) and providing powerful ideas to explain the biological world around them. A list of materials, teacher background information, assessment, standards, safety, and references are provided. This book selection also includes the Table of Contents, Foreword, Acknowledgments, About the Editors, Contributors, Appendix, Image credits, and Index.