Research targeting improved quantification of human performance identified conceptual shortcomings, motivating development of General Systems Performance Theory (GSPT) and the Elemental Resource Model (ERM) for human performance. These have had profound implications for performance measurement, task analysis, and performance prediction. This paper presents an up-to-date summary of these impacts. Foremost are new task analysis and performance prediction methods that have been successful where direct analytic methods do not exist and where traditional statistical analyses have performed poorly. It is concluded that performance theory is indispensable for meaningful progress in human performance modeling and measurement.