A review is given of the theoretical background and the practical application of gas targets for the production of radionuclides with particle accelerators. This common technique is mainly applied for the production of short-lived isotopes for Positron Emission Tomography (PET):11C,18F,15O, and13N 1. This advantageous technique is also used increasingly for manufacturing other radionuclides, e.g.123I. Typical problems are illustrated by examples from the literature and from our own experience. The problem of gas handling has not been covered in this article.