This chapter is dedicated to failure. It focuses entirely on WEP, the security method originally employed with Wi-Fi LANs and which has now been discredited due to its numerous security weaknesses. It may seem strange to devote so much space to a protocol that will soon be consigned to history. However, an understanding of WEP and its failure modes is very educational as a case study and highlights the areas that need to be addressed for real security. The first half of the chapter looks at the design of WEP and the second half shows why it fails to meet its security goals.