Starting with a brief introduction to Gaussian imaging, it is pointed out that generally the Gaussian image of an off-axis point object suffers from a slight focus error. A Petzval image is introduced which takes this error into account. Because of the fact that the refractive index of a transparent substance varies with wavelength, the image distance and the height of an off-axis point object also vary with it. This introduces chromatic aberrations, which are discussed. The concept of ray and wave aberrations is explained and a relationship between the two is given. It is pointed out that the wave aberrations of a multielement system are additive but the ray aberrations are not. Expressions for the wave aberrations are given when the image is observed in a defocused image plane or when the wavefront is tilted. The aberrations of a rotationally symmetric imaging system are described in terms of a power series or Zernike polynomials, with emphasis on primary aberrations. How the expressions for the primary aberrations of a system change when its aperture stop is shifted are given. Analytical expressions for the aberrations of some simple systems are given as examples. Finally, how the aberrations of a system may be observed is also described briefly. Detailed derivations of the expressions given here can be found in Ref. 1. In practice, a preliminary design giving the layout of a system is carried out based on the Gaussian optics. The primary aberrations are determined from the Gaussian parameters giving an approximate image quality, and the final design is obtained with the aid of a standard computer ray-tracing program. The point-spread and optical transfer functions of the aberrated systems are discussed by the first author separately.