Incidents and threats of terrorist bombings of buildings and installations have led to significant interest for blast mitigation techniques for exterior walls. Building material fragmentation is a major cause of human injury during such events. One of the most common methods of construction in buildings is the use of masonry walls. Masonry provides a fast inexpensive way to construct buildings of various heights; however, these walls are extremely vulnerable to blast pressure resulting in collapse, fragmentation, and severe injury to occupants.Membrane retrofit materials are used to strengthen the wall's resistance to lateral pressure. In actual tests, membrane retrofit unreinforced masonry walls have experienced severe cracking and large displacements without collapse. This chapter presents resistance functions for three different scenarios of membrane retrofit unreinforced masonry walls to lateral pressure. These functions are further coupled with single degree of freedom systems to predict the wall response to blast loads. This chapter gives the structural engineer a practical tool for the design of membrane retrofit masonry walls to resist lateral pressures such as wind, and various blast charges and distances. © 2010 Woodhead Publishing Limited. All rights reserved.