This chapter discusses the core characteristics that should be taken into account when selecting pain phenotypes. A pain phenotype may be defined as a measure that directly or indirectly reflects the processing of parts or the whole of the pain system, excluding tissue pathology and pain expression. A very important and necessary characteristic of a pain phenotype is its ability to show variation between subjects. Pain sensitivity has normally been measured in experimental pain models, which have the great advantage of complete control over the noxious stimulus. However, the concept is equally applicable to clinical pain. Heritability can be taken as an indicator of the quality of the phenotype and its suitability for genetic association studies (GAS). Future GAS are likely to demand large samples with in-depth phenotyping of both clinical and experimental pain. Careful selection of phenotypes will be crucial for gaining significant and meaningful results from these studies.