Background: Much confusion exists about the underachievement of gifted students due to significant variations in how the phenomenon has been identified. From a review of the literature, five methods were found to be commonly used to identify gifted underachievement. Aims: The purpose of the study was to assess the equivalence of the commonly used methods to identify gifted underachievement, and to determine which of these methods may be optimal. Sample: Data were collected from a school in Sydney, Australia. Method: Three measures of convergence (i.e., difference in proportions, phi association, and kappa agreement) were used to assess the equivalence of the identification methods, while latent class analysis was used to determine the optimal identification method. Results: The convergence evidence suggested that the commonly used identification methods may not be considered convergent, while the criterion evidence indicated that one of the five identification methods may have strong levels of criterion validity. Conclusions: A conclusion was reached that the simple difference method may be the most valid method to identify gifted underachievement.