This paper investigates the concept of low energy building, examines how urban density might affect building energy design in high density cities, and discuss the strategies for low energy building design in densely populated areas. Using Hong Kong as a case study, the characteristics of dense building development and the major factors affecting low energy building are evaluated. The approaches taken in Hong Kong to accommodate its growing population provide an interesting background for understanding the constraints and potential of low energy design. The energy situation in I-long Kong is explained and the major considerations for energy efficiency in high density conditions are discussed. It is hoped that the research findings would contribute useful information for designing methods to achieve low energy building in a complex and dense urban environment.