The special collection on Research on Selected Aspects of Energy Efficiency of Building Systems is available in the ASCE Library at https://ascelibrary.org/page/jaeied/energy_efficient_building.Given the significant energy loss in buildings, ways that identify effective retrofit measures must be explored. According to Fernandez et al. (2017), retuning can be an effective retrocommissioning of buildings that will revise building control systems for energy savings. While there is consensus and demand to retrofit building systems for improved energy efficiency, currently most of the buildings for which such an investment can be justified seem to be larger commercial buildings. Therefore, special effort is needed to address the case of smaller buildings, such as developing energy management software package tools (Granderson et al. 2017).While making mechanical and electrical systems more efficient will lead to energy saving, because most of the energy loss is through building envelope systems, effective methods are also needed to minimize heat loss through the enclosure by retrofitting exiting façades to be more energy efficient. Regardless of the type of envelope system, control of environmental effects, including heat, air, moisture, and noise transfer through the envelope, must be considered at the design stage (Aksamija and Peters 2017).Another aspect of building envelope control layers that affects energy performance is air tightness, not only for the exterior envelope systems but also any penetration in the floors and ceiling that may allow air leakage eventually through the envelope. Accordingly, measures must be taken to ensure that any plumbing, electrical, and HVAC-related penetrations, as well as light fixtures (Na et al. 2017) are all properly sealed to prevent air leakage. Besides highly energy-efficient HVAC, lighting systems, and appliances, as well as building envelope systems equipped with appropriate thermal insulation and airtightness, which are all parameters of a high-performance building, still other measures that will help minimize energy consumption include passive solar design strategies (McDonald and Chakradhar 2017) and use of renewable energy sources.For a long time, dominant conversation has been around minimizing energy use by providing building envelope systems with large thermal resistance and making mechanical and electrical systems along with appliances highly energy efficient. Yet another perspective is to generate all the electricity needs through solar photovoltaic and other renewable energy sources locally. The resulting zero-net-energy buildings will generate electricity at the site, use it as needed, and store or sell the excess produced electricity in batteries or to the grid. In such systems, weather conditions will affect the performance (Bruggmann and Henze 2018) and during certain periods there will be more demand from the grid, while at other times there will be significantly more excess electricity produced that can be fed back to the grid after the solar-generated DC power is converted to AC power through an inverter.For a complete year-round energy-saving strategy, in addition to efforts on energy efficiency and minimizing energy use and preventing heat loss during the heating season, there is also need in making cooling systems more efficient. In particular, there are concerns about the effect of global warming on outside temperature, which in turn may increase cooling load (Rios et al. 2017). As part of the energy performance evaluation of buildings, weather data (e.g., historical as well as real-time solar radiation) will be needed. In areas where such data is not available, irradiation prediction models may be used, which should be updated based on experimental studies (Do et al. 2021).This Special Collection includes technical papers and case study papers that address in great detail most of the issues discussed in this introduction.References Granderson, J., E. Hult, S. Fernandes, P. Mathew, and R. Mitchell. 2017. “Unlocking energy efficiency in small commercial buildings through mechanical contractors.” J. Archit. Eng. 23 (1): C4016001. https://doi.org/10.1061/(ASCE)AE.1943-5568.0000225. Na, R., S. Lin, Z. Shen, and L. Gu. 2017. “Case study of quantifying energy loss through ceiling-attic recessed lighting fixtures through 3D numerical simulation.” J. Archit. Eng. 23 (1): C5016003. https://doi.org/10.1061/(ASCE)AE.1943-5568.0000226. Rios, F. C., H. Naganathan, W. K. Chong, S. Lee, and A. Alves. 2017. “Analyzing the impact of outside temperature on energy consumption and production patterns in high-performance research buildings in Arizona.” J. Archit. Eng. 23 (3): C4017002. https://doi.org/10.1061/(ASCE)AE.1943-5568.0000242.