Wireless Metropolitan Area Networks (WMANs) provide wireless communications at acceptable bandwidth over much larger geographical areas compared to Wireless Local Area Networks (WLANs). Also known as Wireless Local Loop (WLL), WMANs are based on the IEEE 802.16 standards with commercial name Worldwide Interoperability for Microwave Access (WiMAX). With its global market growing eighty-five percent in 2010 to 1.7 billion U.S. dollars, WiMAX is becoming a major competitor among the prevailing wireless communications technologies. While improved IEEE 802.16 standards and amendments were published and adopted in almost every year of the past decade, existing standards still contain a number of security vulnerabilities inherent from deprecated versions. This chapter starts with an introduction to and overview of Metropolitan Area Networks (MANs), WMANs, WiMAX and IEEE 802.16 standards, then discusses the technical details of WiMAX and IEEE 802.16 security aspects such as confidentiality, integrity, key generation and management, as well as security vulnerabilities, treats, and countermeasures.