This monograph studies research conducted for the purpose of investigating the growth of recognition vocabulary during the early and middle elementary school years in relation to the development of morphological knowledge. The children were tested on a selection of main entry words from a recent unabridged nonhistorical dictionary by means of definition, sentence, and multiple-choice questions. The focus of the present study, however, was on the contribution made by different morphologically defined word types and by knowledge of morphology and word formation to total recognition vocabulary at different age and grade levels. The findings suggest that lexical development can be characterized in terms of increasing morphological complexity. Further, it was found that the proportion of known complex words for which there was evidence that children figured them out by analyzing their morphological structure increased with age and grade.