Introduction to high-energy gamma-ray astronomy
  The present issue is the first of of a two-volume review devoted to gamma-ray astronomy above 100 MeV which has witnessed considerable progress over the last 20 years. The motivations for research in this area are explained, the follow-on articles of these two thematic issues are introduced and a brief history of the field is given. 
Bernard Degrange, G\'erard Fontaine
astro-ph.HE astro-ph.IM