The energy efficiency services sector (EESS) is poised to become an increasingly important part of the U.S. economy. Energy supply and climate change concerns, volatile and increasing energy prices, and a desire for greater energy independence have led many local, state and national leaders to support an increasingly prominent role for energy efficiency (EE) in U.S. energy policy. The national economic recession has also helped to boost the visibility of energy efficiency as part of a strategy to support economic recovery. One of the paradoxes of energy efficiency is the growing consensus among policymakers as to its importance as a low-cost, environmentally-benign resource juxtaposed with the fact that energy efficiency is not a distinct, well-defined industry that is easy to characterize. The growth in public support for and spending on energy efficiency combined with increased investment by private sector market actors will require a significant expansion of the energy efficiency services sector workforce. Trained personnel will be needed to design, implement, manage and evaluate energy efficiency programs and to design, construct, install, and maintain efficient building systems. Bottlenecks may occur if the EESS workforce is unable to expand at the same pace as the increased demand for energy efficiency services. Given the growing interest in energy efficiency, there is a concern among policy makers, program administrators, and others that there is an insufficiently trained workforce in place to meet the energy efficiency goals being put in place by local, state, and federal policymakers. To understand the likelihood of a potential workforce gap and appropriate response strategies, one needs to understand the size, composition, and potential for growth of the EESS. We use a bottom-up approach based upon almost 300 interviews with program administrators, education and training providers, regulatory staff and a variety of EESS employers, trade associations and unions; communications with over 50 sector experts; as well as an extensive literature review. We attempt to provide insight into key aspects of the EESS by describing the current job composition, the current workforce size, our projections for growth in spending and employment in the EESS through 2020, and key issues that may limit this growth.