The effects of heat stress on common bread wheat yield are not always obvious, largely because they are so common and often confounded with other factors. However, high temperature is one of the most important factors affecting wheat yields. There is great potential in the utilization of marker assisted selection to improve heat tolerance. However, successfully identifying QTL has been limited by accurate and dependable phenotypic data that is not confounded by extraneous effects. This study was conducted to provide an in-depth evaluation of 16 contrasting recombinant inbred lines. Using a variety of physiological parameters which directly and indirectly measure heat tolerance, the primary objective of this study is to more accurately characterize the magnitude of heat tolerance expressed by each line under both controlled and field conditions. In absence of drought stress, the effects of exposing each RIL to three consecutive days of 37/27^oC at five days post-anthesis followed by six days of 20/15^oC were studied using growth chambers. Under irrigated field conditions, polycarbonate shelters where used five days post-anthesis to impose higher temperatures over plots containing all RILs. Both studies were replicated 3 times and included control treatments. Chlorophyll a fluorescence, plasma membrane damage, CO₂ fixation, relative water content, transpiration and leaf temperature were recorded for both studies. Under controlled and field conditions these parameters were useful in identifying heat tolerance and susceptibility among the RILs. Increased tolerance was characterized primarily by less damage to photosystem II, reduced plasma membrane damage and higher CO₂ fixation. The physiological parameters used in this study combined with agronomic data successfully identify heat tolerance in wheat under controlled and field conditions thus providing a dependable phenotypic basis for further QTL studies.