Middle and secondary science textbooks have long been a primary support for instruction, often leading to heavy emphasis on domain-specific vocabulary. A longitudinal study was conducted to compare vocabulary loads of science textbooks published since 2010 to results of two previous studies going back to 1983. In each study, textbooks chosen represent convenience samples of commonly used middle and secondary science textbooks. Vocabulary emphasis in science textbooks remains high, and may present a problem for struggling readers and English language learners (ELL). Some vocabulary loads were found to approach or even exceed recommended limits for vocabulary instruction in modern foreign language courses. Also, science vocabulary can present an additional challenge for native English speakers and ELLs since many terms have no common colloquial English or native language equivalents. Recommendations for how to promote effective science vocabulary development are provided. Overall, over three decades there has been little change in heavy emphasis on vocabulary in middle and secondary science textbooks.