Complex systems are all around us. Some were created by nature, such as ant colonies, gene networks, ecosystems, and brains. Others were created by us, such as political systems, stock markets, language, technology, and cities. This introductory chapter to An Introduction to Complex Systems establishes key themes and terminology that are often used to study complex systems, covers the philosophical and scientific underpinnings of the field, and introduces the major groups and researchers who have advanced the field. Along the way examples are used to illustrate more abstract ideas. Advice is also provided on how the reader might navigate the text. The chapter concludes with questions for either reflection or group discussion as well as resources for further exploration.