Walking is a common activity of daily life and researchers have used the range 3-6 km.h[superscript -1] as reference for walking speeds habitually used for transportation. The term self-selected (i.e., individual or comfortable walking pace or speed) is commonly used in the literature and is identified as the most efficient walking speed, with increased efficiency defined by lower oxygen uptake (VO[subscript 2]) per unit mechanical work. Self-selected speed (SS) should be determined using a consistent protocol to ensure an individual's representative walking speed is identifiable and subsequently applicable in a walking project. Controversy exists regarding whether walking tests should be performed on a treadmill or over ground. While some studies using the two methods have found significant differences in energy and performance, others have reported equivalence in these measures. Both methods have advantages. Treadmill testing, for example, allows easy concurrent measurement of physiologic data, such as gas exchange; overground walking requires minimal equipment, more closely replicates everyday walking, and is clinically applicable. However, many of the comparisons reported involved older adults or rehabilitation patients who had cardiac or pulmonary disease, most with limited activity levels and exercise capacity. Indirect calorimetry is often used to measure energy expenditure during treadmill walking. However, assessing individual and group differences in metabolic energy expenditure using oxygen uptake requires individuals to be comfortable with, and able to accommodate to, the equipment. To ensure the data from a treadmill walking protocol does not simply reflect an adaptation to the test, participants traditionally are familiarized with walking on the treadmill before testing. Participants with insufficient exposure to the apparatus may be unable to make the necessary adjustments to achieve a stable and consistent gait pattern, which introduces bias to the measurements. The literature is not consistent about the length of this exposure, with wide variability in exposure time reported, such as 15-20 s, 2 min, 2-3 min, 5 min, and 8 min. Despite the reported variability in time, Morgan et al. (1997) showed that the majority of gait adjustments for young children occurred during the initial 10-min walking period or the familiarization sessions. The results of the study conducted by the authors show no significant differences between SS determined overground and on a treadmill. The mean difference between overground and treadmill step length was 0.06 m. This value, despite being statistically significant, has no clinical importance. (Contains 4 figures.)