Dong-Whee Yang. 2003. Optionality, Output effects and the EPP. Studies in Modern Grammar 32, 43-68. This paper claims that optionality is part of the `optimal design` for natural language in the sense that it is bound to induce some `effect on outcome` which should also be some necessary part of natural language. Specifically, it is argued that the optionality in the Korean Case system, an `imperfection` in the minimalist sense, is justified as an `apparent imperfection` in terms of output effects, or Int effects (Chomsky 2000), which are shown to be two types: A-type and B-type. The A-type Int effects are `week` like specificity, definiteness, thematicity, etc., being interpreted at the canonical Specs, whereas the B-type Int effects are `strong` like focus, specialized semantic function. etc., being interpreted at the non-canonical Specs. The A-type Int effects are claimed to be due to lexically-posited EPP-features whereas the B-type Int effects are claimed to be due to derivationally-introduced EPP-features. Lexically-posited EPP-features are either obligatory or optional whereas derivationally-introduced EPP-features are only optional. It is shown that the interface strategy in terms of output Int effects is superior to Hiraiwa`s (2002) ф-over-ф generalization in the accounts for obligatoriness of the EPP-feature.