When a project involves changing the elevations on a building site, retaining walls are often the answer. Here, landscape contractor Eric Nelson discusses the pros and cons of four different retaining-wall materials: modular block, concrete, timber, and natural stone. When should you have an engineer design your wall? And when is terracing a better option than one tall wall? Nelson answers these common questions, and more. Of course a retaining wall is only as good as the base it is built on, the way it is backfilled, and the drainage incorporated into the design. Drawings offer a look inside well-designed retaining walls, and for each material the author discusses affordability, durability, and complexity of construction.