Reports about excessive Internet use, possibly amounting to an addiction, have increased. Progress with research and treatment of this phenomenon requires valid standardized assessment instruments. A frequently used questionnaire is the Internet Addiction Test (IAT) by Kimberly Young. The 20-item questionnaire is well established in a number of languages, but a German validation was lacking so far. An online (ON) sample (n=1,041, age 24.2±7.2 years, 46.7 percent men) completed an Internet version of the IAT and a student sample (offline [OF] sample, n=841, age: 23.5±3.0 years, 46.8 percent men) filled in a paper/pencil version. The participants also answered questions regarding their Internet use habits. A further sample of 108 students (21.5±2.0 years, 25.7 percent men) completed the questionnaire twice to determine the 14-day retest reliability. The internal consistencies were α=0.91 (ON) and α=0.89 (OF). Item-whole correlations ranged from r=0.23 to r=0.65 (ON) and from r=0.30 to r=0.64 (OF). Two-week retest reliability was r(tt)=0.83. Factor analyses with Varimax rotation yielded the same two factors in both samples, which explained 46.7 percent (ON) and 42.0 percent (OF) of the variance. The IAT score correlated with the time spent in the Internet in a typical week (ON: r=0.44; OF: r=0.38). The German version of the IAT was shown to have good psychometric properties and a stable two-factorial structure. Correlations with online time were in line with those reported for the IAT in other languages.