Adults are generally proficient in their native language, effortlessly processing linguistic information. Nonetheless, individual differences in lexical processing, particularly in word recognition, exist even among skilled readers. In this chapter, I reviewed some interesting studies on individual differences in Korean lexical processing and introduced some effective assessment tools such as the Korean Author Recognition Test (KART) and the bisyllabic onset test to measure individual differences in language processing among Korean adults. Multiple studies have shown that individual differences in language proficiency or word knowledge affect lexical access in Korean, even among native speakers. These results have implications for how crucial individual differences in language proficiency should be considered with respect to Korean word recognition.