Donald Kirkpatrick published a series of articles originating from his doctoral dissertation in the late 1950s describing a four-level training evaluation model. From its beginning, it was easily understood and became one of the most influential evaluation models impacting the field of HRD. While well received and popular, the Kirkpatrick model has been challenged and criticized by scholars, researchers, and practitioners, many of whom developed their models using Kirkpatrick's theoretical framework. This article reviews several evaluation models, and also presents empirical studies utilizing the four levels, collectively demonstrating the usefulness of, but the difficulty in, implementing all four levels.