Vocabulary.com is an online adaptive game that teaches students words by systematic exposure to a variety of question types and activities and by understanding all the word meanings and nuances as they are learned. Vocabulary.com teaches 14,000 words using over 208,000 questions. It has been implemented with the Houston Independent School District's (HISD) fifth through twelfth grade students since the 2014-2015 school year. This evaluation used a quasi-experimental design to determine how Vocabulary.com impacted students' performance on the 2018 State of Texas Assessments of Academic Readiness (STAAR) Grades 3-8 reading and English 1 End-of Course (EOC) exams. Key findings include: (1) The correlation between the number of words students mastered and questions answered correctly on Vocabulary.com ranged from 0.78 to 0.96 for the study sample. On average, the ratio of questions answered correctly to words mastered was 18:1; (2) The highest proportion of eighth- (97.6%) and ninth-grade (95.7%) students who met the Approaches Grade Level standard on the 2018 STAAR 3-8 reading and English I EOC exams, respectively, mastered at least 100 words on Vocabulary.com; (3) Most sixth- to eighth-grade students who showed gains (78.0-86.3%) on the 2018 STAAR 3-8 reading scale scores did not master any words on Vocabulary.com; (4) Treatment effects showed that, on average, sixth- to ninth-grade students in the sample who used Vocabulary.com met the Approaches Grade Level standard on the 2018 STAAR 3-8 reading and English I EOC tests without mastering any Vocabulary.com words; and (5) The strongest predictor of students' performance on Vocabulary.com on the sixth- to eighth grade 2018 STAAR reading was their prior (2017 STAAR 3-8) reading scale score. They predicted between 50 to 62 percent of the variance in students' scale scores.