The investigation of software engineering techniques by using the experiment sets up the discipline of experimental software engineering. The effectiveness and efficiency of software engineering techniques have been studied in experiments and confronted with empirical data. The results of these experiments are published in a diversity of journals and proceeding papers. However, there is not an overview so far which represents the available results systematically. By taking the results from published experiments that deal with analysis, design, implementation, test, maintenance, quality assurance, and reuse techniques a preliminary software engineering theory is developed. From this theory, fruitful problems, suggestions for gathering new data, and entirely new lines of investigation are deduced.