Gamma-ray astrophysics: roots, growth, and success

This short review outlines the historic roots of high-energy astronomy in the early twentieth century with the discovery of radioactivity, cosmic ray research, and the development of detectors to measure high-energy photons. The beginning of the space age in the 1960s provided the means to observe cosmic gamma rays unhindered by the absorption and local background inside Earth's atmosphere. We describe the results from pioneering missions in the 1970s up to the first 'golden age' of gamma-ray astronomy in the 1990s with the Compton Gamma-Ray Observatory (CGRO). The focus in this review will be on gamma\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gamma$$\end{document}-ray astronomy in the pair creation energy band, above several 10's of MeV.