This paper presents experimental data on the processing of relative and adjunct clauses in Korean. Both types of clause contain null elements (gaps), although these gaps may possibly represent different types of empty categories (deletion under movement in relative clauses, null pronominals in adjunct clauses). The experimental evidence shows that even in head-final languages like Korean, subject gaps of all types enjoy a processing advantage over object gaps, thus adding support to the idea that the subject advantage is a universal principle of grammar. Different explanatory mechanisms have been proposed to account for this subject advantage. The Korean data support structurally-based accounts rather than accounts that rely on the linear distance between the filler and the gap in a long-distance dependency. Theoretically, these new data shed additional light on—but do not entirely resolve—long-standing controversies over whether Korean relative clauses represent English-like operator-movement structures or structures with an unselectively bound null pronominal.