Following an extensive overview of the subject, this study explores the relationships between second-language (L2) explicit/implicit knowledge sources, embedded in the declarative/procedural memory systems, and L2 working memory (WM) capacity. It further examines the relationships between L2 reading comprehension and L2 WM capacity as well as those between L2 reading comprehension and L2 explicit/implicit knowledge sources. Participants were late adult learners of English as an L2, with a relatively advanced level of English proficiency. They completed tests measuring their WM capacity, explicit knowledge, implicit knowledge, and L2 reading comprehension. Correlation analysis revealed significant relationships between L2 WM capacity and both explicit and implicit L2 knowledge. Exploratory factor analysis showed that explicit knowledge, WM capacity, and L2 reading comprehension loaded on a single factor whereas implicit L2 knowledge formed an independent factor with no relationship to L2 reading. The results suggest that L2 WM is able to manipulate and store both explicit and implicit L2 input through controlled and automatic processes. They also suggest that L2 explicit knowledge, connected with the control processes of the declarative system's lexical/semantic features, and L2 WM, reflecting attentional resource capacity/allocation associated with control processes, play an important role in L2 reading comprehension.