How do you draw a straight line? How do you determine if a circle is really round? These may sound like simple or even trivial mathematical problems, but to an engineer the answers can mean the difference between success and failure. This website contains interactive materials to support the book How Round Is Your Circle?, by John Bryant and Chris Sangwin. This book invites readers to explore many of the same fundamental questions that working engineers deal with every day. It also illustrates how physical models are created from abstract mathematical ones.