A conceptually simple high speed infrared (IR) zoom lens system has been designed to operate in the 8 to 13µm wavelength region for the detection of missile signatures. Even though the optical system is designed to provide high resolution throughout the 3:1 zoom range at a large relative aperture, it is extremely lightweight and compact. The zoom concept has been derived from first-order optical principles which will be presented as the starting point of the optical system design. The use of the symmetry principle for aberration correction will be discussed. Paraxial ray trace was used as an aid in establishing lens powers and groupings in the initial layout. Considerations which governed material selection will also be discussed. The zoom system consists of six lens elements including a field flattener near the image plane. The total glass weight is only 172.4 grams and the length is 161.79 mm. There are two moving lens components linked together with a cam to provide mechanical compensation. The detail design was accomplished through the use of a computer optimization program. The 1 mrad image resolution requirement is achieved throughout the zoom range. Performance data will be presented and tolerance considerations will be discussed.