This chapter reviews previous work on cross-layer design (CLD) optimization for wireless local area networks (WLANs) and discusses the motivation for using CLD optimization. It also discusses the strengths and weaknesses of CLD methods. The chapter categorizes CLD methods based on open systems interconnection (OSI) layering approaches. CLD can be defined in several ways. In traditional layered architecture, direct communication between nonadjacent layers is not permitted, and communication between adjacent layers is limited to procedure calls and responses. As a novel idea for network design, CLD appeared nearly a decade ago for improving the performance of wireless networks. W. Ge et al. proposed a CLD scheme to provide reliable and efficient multicast wireless communications. P. P. Pham et al. proposed a CLD approach that employs the predictability of Rayleigh fading channels to improve the performance of ad hoc networks. Conti et al. proposed a CLD architecture called MobileMan to optimize the performance of mobile ad hoc networks.