Penry, JT, Wilcox, AR, and Yun, J. Validity and reliability analysis of Cooper's 12-minute run and the multistage shuttle run in healthy adults. J Strength Cond Res 25(3): 597-605, 2011-Field tests are a practical method to assess aerobic fitness, but they demonstrate greater error variability than laboratory tests. The principal goals of this study were to identify potential sources of systematic error in 2 commonly used field tests (Cooper's 12-minute run [12MR] and the multistage shuttle run [MSR]) and estimate the reliability of the 2 tests from these data. In addition, criterion-related validity evidence for field tests was evaluated via Bland-Altman plots. To assess trends across test protocol and test trials, 60 subjects (mean age = 21.8 ± 3.6 years) completed 6 test trials, including 3 trials of each field test. Of these 60 individuals, 21 volunteers completed an incremental treadmill run and expired gas analysis (TR) that was used to establish criterion-related validity evidence for the 2 field tests. G-study analysis of the field test data returned a high reliability coefficient (ϕ = 0.96), with the largest amount of systematic error variance (4.3%) attributable to an interaction between subjects and test occasions. The MSR predicted JOURNAL/jscr/04.02/00124278-201103000-00004/ENTITY_OV0312/v/2017-07-20T235437Z/r/image-pngo2max scores lower than those measured in the laboratory setting (p < 0.01), whereas 12MR and TR scores were not different (p > 0.05). However, Bland-Altman plots showed the 12MR to underestimate JOURNAL/jscr/04.02/00124278-201103000-00004/ENTITY_OV0312/v/2017-07-20T235437Z/r/image-pngo2max scores at lower JOURNAL/jscr/04.02/00124278-201103000-00004/ENTITY_OV0312/v/2017-07-20T235437Z/r/image-pngo2max values and overestimate JOURNAL/jscr/04.02/00124278-201103000-00004/ENTITY_OV0312/v/2017-07-20T235437Z/r/image-pngo2max scores at higher values, a trend not observed in the MSR data. These data suggest high overall reliability for JOURNAL/jscr/04.02/00124278-201103000-00004/ENTITY_OV0312/v/2017-07-20T235437Z/r/image-pngo2max field tests in young, healthy individuals. Nevertheless, test administrators must use caution when attempting to use field test data to predict criterion JOURNAL/jscr/04.02/00124278-201103000-00004/ENTITY_OV0312/v/2017-07-20T235437Z/r/image-pngo2max scores. The MSR appears to be a more useful tool than the 12MR because of a consistent mean bias across fitness levels.