Zoom lenses generally contain moveable lens groups. The behavior of one moveable lens, two moveable lenses, and two moveable lenses together with one fixed lens is discussed in terms of the Delano and Aperture diagrams. This is followed by a discussion of the important features of the two diagrams with respect to zoom lenses: stop shift, partial vignetting, sensitivity to cam mechanism, aberrational behavior of thin lenses, additional aberrations of aspheres. Then the most widely used zoom lenses in photography and cinematography, the standard mechanically compensated zoom lenses, are presented. The four principal types are explained in terms of the diagrams and are discussed with respect to vignetting, aberrational behavior and sensitivity to cam mechanism. Further on simple zoom lenses which contain only a two-lens zoom kernel and a relay lens are considered. Two main types exist, each of which can be divided into three different subgroups. It is true that the Delano diagram offers an impressive and quick overall view of the features of a zoom lens. But designing a new zoom lens from scratch requires a little more than merely looking at beautiful diagrams. What can and must be done is demonstrated by the following design example of a simple zoom lens.