Many explicit or implicit requirements on MANETs are mutually exclusive. Implementing such a net always requires decisions and trade-offs. Mobile Ad-hoc NETworks (MANETs) rely on cooperation of all participating nodes. Thus they are vulnerable to selfish nodes using the net without providing own resources, as well as malicious nodes attacking the net infrastructure. This paper outlines important attacks and summarizes popular approaches to design secure MANET protocols in order to detect selfish and malicious nodes and to enforce cooperation.