An important goal of most optical systems is to form an image of an object. Telescopes, microscopes, cameras and lens systems for lithography are amongst the many examples. This course gives insight into the basic principles to design, evaluate and optimize image forming optical systems. Due to the prevalence of micro-optics in many applications we will also touch upon diffraction and diffractive imaging elements. We will also hightlight aspects of fabrication and tolerancing because ever increasing demands on the image quality and miniaturization of optical systems pushes designers to take up those aspects as early as in the conceptual design phase.