Reading research has recognized the strong relationship between vocabulary and reading comprehension. However, we are still perplexed by the precise nature of how readers access and retrieve word meanings while reading. We have not reached a consensus on "what it means to know a word" (e.g., Anderson & Nagy, 1991; Nation, 2001) or how to assess vocabulary knowledge. While background knowledge influences our interpretation of word meanings (e.g., Kintsch, 1998; Adams, 1994), it is unclear how many studies have considered its role in second language reading and vocabulary acquisition. This narrative synthesis integrates empirical findings on the issue, investigating how vocabulary has been conceptualized in the field and what relationships have been explored between L2 vocabulary, prior knowledge, and reading comprehension. Vigorous criteria were used to select 15 studies for inclusion in this synthesis. The majority of studies investigated vocabulary size or depth as a direct causal variable of reading comprehension. Conceptualizations of vocabulary depth knowledge include aspects of prior knowledge, but investigations of depth are limited by available assessment tools. Future research will benefit from investigating bidirectional relationships between prior knowledge, reading, and vocabulary learning, and consider more innovative techniques to investigate prior knowledge in vocabulary and reading.