One way to understand the climatic exposure and identify the critical aspects of a wall system is to examine it from the viewpoint of the rain. For a wall to leak, water must reach and penetrate the exterior surface. It then must either penetrate through the thickness of the wall or travel down and leak through at openings or intermediate floor levels. Preventing leaks means stopping water at at some point along its path. Checkpoint #1: Keeping water off the wall surface Checkpoint #2: Keeping water from penetrating the surface Checkpoint #3: Keeping the wall from cracking Checkpoint #4: Keeping water out at interfaces between masonry and nonmasonry elements Checkpoint #5: Keeping water from penetrating through the thickness of the wall Checkpoint #6: Directing water's path with flashing Masonry walls long have and will continue to perform under wind-driven rains. But the more we understand about how these wall systems work, the fewer problems we will have.