The importance of IEEE-Standard 802.16 (WiMAX) is growing and will compete with technologies such as UMTS. However, acceptance and adoption of technologies and services also depend on security. Therefore, this paper will deal with some of the basic security features of IEEE 802.16d (fixed WiMAX). It will summarize the most important elements of the security architecture, present some of its weaknesses, potential attacks and viable counter measurements. Furthermore, the authors will introduce the basic improvements made by the IEEE 802.16e standard (mobile WiMAX) compared to Fixed WiMAX.