Connections can be physical, as with bridges, or immaterial, as with friendships. Both types of connections can be understood using the same mathematical framework called network theory, or graph theory, which is a way to abstract and quantify the notion of connectivity. This unit looks at how this branch of mathematics provides insights into extremely complicated networks such as ecosystems. Mathematics Illuminated is a 13-part multimedia learning resource for adult learners and high school teachers in math and other disciplines. The series explores major themes in the field of mathematics, from mankind's earliest study of prime numbers to the cutting edge mathematics used to reveal the shape of the universe. Rather than a series of problems to be solved, mathematics is presented as play we engage in to answer deep questions that are relevant in our world today. Mathematics also provides us with a powerful language for uncovering and describing phenomena in the world around us. The groundbreaking videos, interactive Web exploration, text materials, and group activities included in Mathematics Illuminated reveal the secrets and hidden delights of the ever-evolving world of mathematics.