Poultry production primarily focuses on the supply of saleable products. Meat and eggs are classified as production traits and are the focus of improvement in poultry breeding organizations, while the functional traits of birds have received much less attention. However, over the past decade, the decline in some functional traits and increased awareness of poultry welfare has led to increasing interest in including functional traits in genetic improvement programmes. In this chapter, we first consider the key functional traits of reproductive capacity, skeletal integrity, cardiovascular fitness and disease resistance. We review how advances in molecular biology and sequencing of the chicken genome have made it possible to identify quantitative trait loci (QTLs) and gene variants associated with functional traits, and discuss how these traits are genetically related to production traits. We consider how this information can be used to implement sustainable genetic improvement of poultry.