Recently, there has been a burgeoning interest in relationship between reading comprehension and vocabulary knowledge, and this emanates from fact that academic achievement is closely related to reading performance (Adamson, 1993; Collier, 1989). As might be expected, best way to learn new vocabulary items is considered to read, and knowing extensive vocabulary is a prerequisite to understand a text (Eskey, 2005). Therefore, it is necessary to scrutinize breadth of a person's vocabulary as a predictor of reading comprehension on a regular basis to reach a consensus about amount of vocabulary needed by an L2 learner for a reasonable comprehension of expository texts. However, although there have been several studies evaluating relationship between reading comprehension and vocabulary knowledge in general, these studies do not include Turkish learners of English as participants whose native language is not etymologically related to English. Scholars, on other hand, acknowledge that English as a foreign language (EFL) learners feel burden of reading in an L2 twice as much as their counterparts, L1 readers, do, and success is difficult to come by without being a skilled reader (Carrell & Grabe, 2002). For that reason, present article aims to investigate a perennial concern, that is, relationship between vocabulary coverage and reading comprehension of a group of Turkish EFL learners because first reading is an indispensable skill for academic achievement and second reading comprehension is directly linked to learners' vocabulary knowledge. After a brief summary about reading comprehension and vocabulary knowledge as a reciprocal process, article aims to discuss what percent of vocabulary items should be known in an expository text to comprehend it, and corresponding vocabulary size will be discussed by analyzing sizes suggested in studies of Nation (2006), and Laufer and Ravenhorst-Kalovski (2010).A Reciprocal Process: Reading Comprehension and Vocabulary SizeReading is delineated as the process of receiving and interpreting information encoded in language form via medium of print (Urquhart & Weir, 1998, p. 22). As stated by Linan-Thompson and Vaughn (2007), and Grabe and Stoller (2002), reading comprehension is main purpose for reading, and this purpose underlies and supports most of other purposes for reading. However, since reading involves cognitive processes, reading comprehension is an invisible concept that can only be inferred (Bernhardt, 2011). Moreover, reading comprehension presents some challenges for learners as many students consider reading a boring and difficult task. When complexity of reading is considered together with its purposes and properties, it becomes clear that reading is complex for both teaching and learning.In addition to complexity of reading, it is unarguably clear that relationship between vocabulary size and reading comprehension is reciprocal. While some scholars focus on effects of vocabulary size on reading comprehension in their studies, some others study effects of reading comprehension on vocabulary size (Eskey, 2005; Hu & Nation, 2000; Nation, 2001; Nation & Angell, 2006). According to Rumelhart (1977) and Stanovich (1980), in evidence-based reading models, bottom-up processes such as word recognition and lexical access go hand in hand with top-down processes such as integrating background knowledge and processing strategies. Readers need automaticity in both word recognition and lexical access (Walter, 2003). From a lexical perspective, Anderson (2009) and DeKeyser (2007) summarize this long learning process as a path from understanding a word's meaning to learning a major meaning of a word, and then learning many aspects of a word's meaning and use. Therefore, faster a reader recognizes a word, which is linked to learners' vocabulary knowledge and automaticity, better reading comprehension will take place. …