Modern commercial transport aircraft contain digital avionics systems for more and more safety critical functions. The reliability and safety of those new systems depend on the hardware reliability and on the reliability of the embedded software. In this report, an overview is given of available methods and techniques to develop reliable software-based avionics systems, especially for safety critical functions. After a description of the differences between analog and digital systems, the policy of the FAA to certify software-based systems is presented. The subsequent chapters deal with methods to minimize the number of errors during software development, methods to remove as many errors as possible via testing and methods to minimize the effect of remaining errors during operational flights. A safety analysis regarding commonmode failures is given, followed by a short ov.erview of reliability related techniques which are used by avionics manufacturers.