Purpose: The purpose of this study was to develop and evaluate measures reflecting the onset of tense marking for children between the ages of 2;0 (years;months) and 3;0. Method: The validity of 4 cumulative measures of tense marker emergence and productivity was evaluated relative to existing measures of early grammatical development in a sample of 20 children followed longitudinally. Fourteen children were at risk for specific language impairment (AR-SLI group), and 6 children had low average language abilities (LA group). Results: All measures of onset were highly correlated with the traditional measures; however, children's progress toward mastery of grammatical tense marking was best explained by the productivity of their tense marking systems. Finally, the onset measures imposing productivity requirements best differentiated children in the LA group from those in the AR-SLI group. Conclusions: The clinical implications for using the late onset of tense marking to improve the early identification of SLI are discussed. (Contains 7 tables, 2 figures, and 1 footnote.)