The human body is made up of trillions of tiny cells that cannot be seen by the naked eye. The functioning units inside these cells are macromolecules that need to travel in the three-dimensional cell-space to distances ten thousand times their size. This movement is highly ordered, requires energy and takes place on molecular tracks that serve as a sophisticated transport systemsomewhat equivalent to the multimodal rail-highway-river networks of large metropolises. All the systems of the human body depend on the efficient delivery of macromolecules to their right destination at the right timeboth within and between cells. Breakdown of this traffic system results in a variety of diseases including diabetes, cancer and heart disease, as well as immunological, neurological and developmental disorders. During the last half a century, scientists have made a quantum leap in unraveling the mysteries of trafficking inside cells. The three sections of this book together cover the past, present and future of this rapidly developing and intriguing field.