The properties and use of thermal insulation to reduce heat loss or gain across the building envelope have been a recognized energy conservation strategy for many decades. Materials available for use as building insulation include naturally occurring fibers and particles, man-made fibers, reflective systems, cellular plastics, evacuated systems, aerogels, and hybrid insulations that combine two or more types of insulation. This chapter discusses the basic theory of insulations and the way they are used and evaluated. Performance limitations are identified and discussion of the performance of building elements that represent combinations of insulation and other building material is contained in this chapter. The impact of air infiltration and moisture on the performance of building insulation systems will be discussed in this chapter. The language associated with thermal insulation technology and key thermal properties have been included to help the reader use the vast literature associated with building thermal insulation. Recent advances in the field of building insulation have been added to this fourth edition of the handbook.