As one of the largest owners and developers of office properties in the United States, we continually seek ways to promote our growth and improve our performance by attracting and retaining tenants and controlling our costs. The efficient operation of our buildings in an environmentally responsible manner and positively impacting the communities in which we operate are important components of this strategy. Boston Properties reviews on an ongoing basis which measures are the most effective in supporting this strategy and where the greatest risk lies in not taking action. We focus our efforts on the areas we can control and make significant impacts. These include the design and construction of our new developments and the operation of our existing buildings. While our sustainability management and investments reach across our portfolio, we have focused our reporting of aggregate performance on office buildings over which we have full control of the operational systems and information and which are at least fifty percent occupied. Office buildings comprise the vast majority of our portfolio and by concentrating on similarly situated buildings we are able to compare performance and judge the effectiveness of our sustainability measures in a more meaningful way. Over time we will continue to assess each property and expand our reporting program, as appropriate.
Our sustainability efforts are centered on energy efficiency, waste reduction, water conservation and maintaining a quality environment for our tenants through our green cleaning program. As a company with a core strategy of long-term ownership, we use high quality building materials and design to reduce replacement costs and repairs and to ensure our buildings remain competitive for the long term. We invest in energy systems which reduce the cost of operations for ourselves and our tenants. A core element of our development strategy is a commitment to the U.S. Green Building Council's LEED® (Leadership in Energy and Environmental Design) certification program. Our Chief Operating Officer oversees a Sustainability Committee with representatives from all of our regions with the following goals:
We have committed to using the ENERGY STAR® Portfolio Manager system to measure the results of our efforts to increase energy efficiency and water conservation. Our Regional Managers and Regional Heads of Property Management are accountable in their annual quantifiable performance goals for the success of their efforts to measure and improve our efficiency as measured on this system.
Our buildings are primarily located in cities which are actively regulating building sustainability performance, particularly as it relates to energy efficiency. By trying to keep ahead of the policy makers through addressing the sustainable operation of our buildings, we demonstrate leadership in our communities and mitigate the risk of being rushed into costly measures to catch-up with new legal requirements. We also mitigate risk by focusing on the supply side of energy costs by entering into contracts to purchase energy in the future at fixed costs for electricity and gas when feasible, in addition to mitigating demand through increased efficiency.
At Boston Properties, we take great pride in our people and our workplace. Our success is a result of the efforts of each of our employees. Our commitments to quality in our workplace, business practices, and in our employee benefits align our employees with our strategy to be successful and, in return, helps them make positive contributions in our communities. We have implemented business responsibility practices, including employee training and education, a comprehensive benefits program, and a wide range of charitable activities. As a result of these efforts, Boston Properties makes a positive impact in the communities in which we live and work.
While we are proud of the results of our efforts to date, we will continue to assess where risk and opportunity lie in order to most effectively focus our program in the future. This report reflects our ongoing commitment to the transparency of our sustainability program and its achievements.