The Comprehension Hypothesis (CH) is the most powerful hypothesis in the field of Second Language Acquisition despite the presence of the rivals the skill-building hypothesis, the output hypothesis, and the interaction hypothesis. The competing hypotheses state that consciously learned linguistic knowledge is a necessary step for the development of second language competence whereas the CH posits that comprehensible input as the crucial ingredient of SLA. Moreover, conscious knowledge of second language can be used only to edit the output of the acquired language. The article further reviews some of the current research on vocabulary acquisition and the review confirms that incidental acquisition of vocabulary is more powerful than intentional learning.