Counting is an act of organization, a listing of a collection of things in an orderly fashion. Sometimes it's easy; for instance counting people in a room. But listing all the possible seating arrangements of those people around a circular table is more challenging. This unit looks at combinatorics, the mathematics of counting complicated configurations. In an age in which the organization of bits and bytes of data is of paramount importance - as with the human genome - combinatorics is essential. 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.