||How can disciplinary research in physiology complement wheat breeding? This introductory chapter is intended to provide broad guidelines to help breeding programs: 1) assess whether physiological criteria should be included in a breeding strategy; 2) evaluate specific physiological selection traits and determine their usefulness in breeding. The other chapters in this book provide more explicit information on how physiological approaches can be used in breeding work for a variety of environmental conditions. Physiological criteria are commonly though not explicitly used in breeding programs. A good example is selection for reduced height, which improves lodging resistance, partitioning of total biomass to grain yield, and responsiveness to management. Another is differential sensitivity to photoperiod and vernalizing cold, which permit adaptation of varieties to a wide range of latitudes, as well as to winter- and spring-sown habitats. Despite a lack of detailed understanding of how photoperiod and vernalization sensitivity interact with each other and the environment, the relatively simple inheritance of photoperiod (Ppd) and vernalization (Vrn) sensitivity genes and their obvious phenotypic expression (i.e. earliness versus lateness) has permitted them to be modified in many breeding programs. The same is true for the height reduction (Rht) gene. In the future an increased understanding of the genetic basis of these traits may enable breeding programs to exploit them further. Selection for reduced height and improved adaptation to environment has had a profound impact on modern plant breeding, and the improvement in yield potential of spring wheat since the Green Revolution has been shown to be associated with a number of other physiological factors (Reynolds et al., 1999). Nonetheless, most breeding programs do not put much emphasis on selecting physiological traits per se (Rajaram and van Ginkel, 1996). Exceptions would include: 1) the staygreen character, which has been selected for in relation to improved disease resistance and is associated with high chlorophyll content and photosynthetic rate in Veery wheats, for example Seri82 (Fischer et al., 1998), and 2) more erect leaf angle, a common trait in many high yielding bread and durum wheat plant types that was introgressed into the CIMMYT germplasm pool in the early 1970s (Fischer, 1996). A recent survey of plant breeders and physiologists addressed the question of how physiological approaches in plant breeding could have greater impact (Jackson et al., 1996). According to the survey, while the impacts of physiological research on breeding programs have been limited in the past, future impacts may arise through: Focusing physiological work on an appropriate range of germplasm (which will depend on the specific breeding objectives); Working with larger populations to enable extrapolation of findings to breeding methods; Identifying traits for use as indirect selection criteria, in addition to those already used in core breeding programs; Identifying traits for use as selection criteria in introgression programs; Conducting selection trials in more representative environments, and Developing tools that could be quickly and easily applied to large numbers of segregating lines. In this and the following chapters, many of these suggestions are incorporated into a research framework for assessing the value of physiological selection traits in a breeding context.