It is Market studies (or market investigation or sector inquiry) that investigates and evaluates how a specific market/industry works. Several antitrust authorities in advanced countries actively and widely conduct its inquiry into a particular sector or market of the economy when there are some evidences indicating that competition may be restricted or distorted. Market study is one of the important antitrust authority tools and is quite complementary to antitrust enforcement. This paper studies how other advanced countries such as US, UK, and EC conduct market studies and finds implications for Korean antitrust authority.