<i>Planck</i>pre-launch status: The optical system

<i>Planck<i/> is a scientific satellite that represents the next milestone in space-based research related to the cosmic microwave background, and in many other astrophysical fields. <i>Planck<i/> was launched on 14 May of 2009 and is now operational. The uncertainty in the optical response of its detectors is a key factor allowing <i>Planck<i/> to achieve its scientific objectives. More than a decade of analysis and measurements have gone into achieving the required performances. In this paper, we describe the main aspects of the <i>Planck<i/> optics that are relevant to science, and the estimated in-flight performance, based on the knowledge available at the time of launch. We also briefly describe the impact of the major systematic effects of optical origin, and the concept of in-flight optical calibration. Detailed discussions of related areas are provided in accompanying papers.