This paper discusses creative elements of deep whole-building retrofits. It discusses process and technique, how to select suitable buildings, the key players that must be involved, project development tasks the design team must complete, and tools and skills that engineers will need to acquire to comprehensively and cost-effectively retrofit the existing commercial building stock. For retrofits to achieve their potential in carbon emissions reductions, design teams must design deeper building retrofits that save more energy. Whole-building retrofits are more than lighting or chiller replacements. Engineers need to expand their skill sets to focus on load reduction measures that allow for efficiency improvements with avoided capital costs, or “tunnel through the cost barrier.” Engineers advise owners on key decisions by estimating the carbon savings and life cycle cost of carefully designed packages of measures. This paper describes tools for wise decision-making that balance predicted operational cost savings with reduced carbon production. In this manner, design teams can achieve deep comprehensive building retrofits that provide significant carbon reduction. Contributing to climate stabilization is one of the largest challenges facing the building industry. Engineers have an expanding opportunity and role to play in rising to this challenge through deep retrofits of existing buildings.This article or paper was published in ASHRAE Transactions, 2010, vol 116, part 2. Copyright 2010 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Reprinted by permission at www.rmi.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Transactions, visit www.ashrae.org.