Construction engineering has an important role in achieving reductions in energy consumption and emission of carbon dioxide (CO2). Governments and the construction industry have actively participated and sponsored research and development to promote principles of sustainability, especially in the fields of using industrial byproducts, reducing waste, reusing building frames, refurbishing buildings and recycling construction and demolition waste. At the same time, engineers are expected to provide value-for-money to the clients. In the UK, they have to be mindful that they could be liable for defects in construction arising from shortcomings in design and specifications of materials that do not comply with the appropriate standards. While accepting the need to use industrial byproducts and recycled materials, they may wish to consider whether usage of these materials can really result in any overall saving in energy and reduction in emission of CO2. This paper examines these issues from an engineering point of view. Satish Desai OBE Visiting Professor, School of Civil Engineering and Construction, Kingston University, London