The Common Core State Standards emphasize the need for U.S. students to read complex texts. As a result, the level of word complexity for primary-level texts is important, particularly the dimensions of and changes in complexity between first grade and the important third-grade high-stakes testing year. In this study, we addressed word complexity in these grades by examining its dimensions and differences in the texts in three widely used U.S. reading programs. Fourteen measures of word complexity were computed, and exploratory factor analysis established that four dimensions--orthography, length, familiarity, and morphology--characterized word complexity. As expected, the third-grade texts have more complex words than the first-grade texts have in the four dimensions, with the greatest differences in length and familiarity. More surprisingly, the words in the first-grade texts increase in complexity over the year, but overall, the words in the third-grade texts do not. Polysyllabic words are common in texts in both grades, comprising 48% of unique words in first-grade texts and 65% in third-grade texts. Polymorphemic words comprise 13% of unique first-grade words and 19% of third-grade words (for derived words, 3% and 6%, respectively, of all words). Results show that word complexity changes markedly between grades as expected, not only in length and familiarity but also in syllabic and morphemic structure. Implications for instruction and future word complexity analyses are discussed.