Reverse engineering IMSI Catchers remains a daunting task due to the overall secrecy surrounding them. In order to detect their presence, its necessary understand how GSM works and how IMSI Catchers exploit GSM weaknesses. I'll describe what a disruption to network topography looks like and how to detect them. This talk will focus mostly on an introduction to IMSI Catchers with discussion of open legal questions related to our FOIA work. Finally we'll see hardware and software solutions for detecting the presence of IMSI Catchers.