Although the primary liability for online copyright infringement may fall on the individual who illegally copies, transfers, and/or distributes the copyrighted content, the issue of indirect liability for Online Service Providers (OSPS) that provide a channel for the distribution of illegal content has recently come under the spotlight. Currently, in an effort to avoid liability for indirect copyright infringement and improve their reputation, most OSPs have voluntarily applied filtering technology. Under the Copyright Act of Korea, special types of OSPS including P2P and Web-based Hard Drive (WebHard) are required to incorporate filtering technology, and may be charged with penalties if found without one. However, despite the clear need for filtering mechanisms, several arguments have been set forth that question the efficacy and appropriateness of the system. As such, this paper discusses the liability theory adopted in the US. -a leader in internet technology development-and analyzes the scope of liability and filtering related regulations in our copyright law. In addition, this paper considers the current applications of filtering as well as limits of the applied filtering technology in OSPS today. Finally, we make four suggestions to improve filtering in Korea, addressing issues such as clarifying the limits and responsibilities of OSPS, searching for cooperative solutions between copyright holders and OSPS, standardizing the filtering technology to enable compatibility among different filtering techniques, and others.