Hydrogen
For many years high resolution spectroscopy on atomic hydrogen has been essential for the development and testing of fundamental theories. Today the precise determination of transition frequencies and relations among them provides one of the most rigorous verification of QED [1,2,3]. The most precise values of the Rydberg constant to date are derived from optical frequency measurements in atomic hydrogen [4,5]. From the comparison of the transition frequencies in hydrogen and deuterium the deuteron structure radius can be extracted. Since the deuteron is the simplest nucleus (besides the proton) knowledge about its structure is essential to test nuclear models [6]. A survey of recent developments in high resolution hydrogen spectroscopy and its theoretical treatment is presented in [7].