Much is known about the impact of vocabulary instruction on reading skills, word knowledge, and reading comprehension. However, knowledge of the underlying theories that guide vocabulary instruction and their potential impact on teachers’ performance and/or students’ achievement has not been investigated. In this content analysis, articles published in The Reading Teacher and Journal of Adolescent and Adult Literacy between 2007 and 2017 were dissected to identify and code embedded word-learning strategies, grade levels addressed, target student populations, and desired outcomes (receptive or productive vocabulary). Our primary goal was to examine the embedded word-learning strategies within the articles, and to identify the theories on which they were built. Findings showed that a combination of theories guided most strategy recommendations: Social constructivism and sociocultural theories, schema and psycholinguistic theories, motivation theory, and dual coding theory. We also parallel-coded our findings with a recent review of literature on vocabulary instruction by Wright and Cervetti (2017), and found that they corresponded with the original coding. Follow-up quantitative studies can use the salient theories detected in this content analysis to investigate whether knowledge of underlying theories has an impact on teachers’ performance and student vocabulary and reading comprehension achievement.