The Injection Laser System (ILS) optical design of the National Ignition Facility (NIF) laser system is described, covering design functions, requirements and constraints, various approaches and options, and the resultant configuration. The front end compromises approximately 70 optical elements per beamline, and 8300 elements total, whose characteristic dimensions are from two to six inches. Individual beamlines span a distance of approximately 15 meters. A variety of optical element types are used: spherical and aspheric lenses, mirrors, polarizers, multi-order waveplates, Faraday rotators, and laser rods. The front end performs multiple functions, namely to: image the pupil of the NIF laser system; amplify the beam's energy with a gain of approximately 104; magnify the beam size by 30x; split the beam four-fold; provide back-reflection isolation; and adjust the pupil location, along with arrival time of the pulse, on a beam-by-beam basis. Due to the high-energy nature of the beam, particular attention is paid to minimizing peak fluence throughout the system, thus reducing the likelihood of optical damage. The front end must deliver wavefront with no more than approximately a wave of P-V aberration (at 1.053 micrometer). This demanding wavefront requirement requires optics' surfaces and transmitted wavefronts to be of relatively high quality, typically 1/10 wave P-V (at 0.633 micrometer).