The purpose of this paper is to review studies in incidental second language (L2) vocabulary acquisition through reading and to show the future directions in this field. First, the paper reviews previous empirical studies within the framework of the quality and quantity of vocabulary processing in L2 vocabulary acquisition which thus far has been widely used mainly in the field of incidental L2 vocabulary acquisition research. This paper also focuses on the methodologies used in previous studies for assessing L2 learners' incidental vocabulary acquisition and points out that most of these studies have been conducted without a theoretical framework which could provide the explanation of the phenomena of L2 vocabulary acquisition. The studies reviewed, therefore, do not inquire about incidental L2 vocabulary acquisition through reading in a systematic manner. Secondly, reviewed here - with a particular focus on the type of processing-resource allocation model (Barcroft, 2000) as a framework to capture the role of the quality of vocabulary processing - are some recent studies which have attempted to offer theoretical accounts of L2 vocabulary acquisition. The paper also reviews some theoretical accounts of the role of the quantity of vocabulary processing which were originally proposed in cognitive psychology and argues that these accounts can be utilized in incidental L2 vocabulary acquisition research. Finally, based on these reviews, this paper discusses the future directions of incidental L2 vocabulary acquisition research.