For the realization of precision medicine in cancer treatment, discovery, and validation of clinically useful biomarker is the most important prerequisite. Biomarkers are needed and used for evaluation of cancer susceptibility, cancer screening (early detection), cancer subtyping, prediction of prognosis, decision of appropriate adjuvant therapy and duration of therapy, and for monitoring of recurrence. Biomarkers are also needed for decision of target therapy in metastatic cancer and monitoring of their response during follow-up. Now is the era of Next-Generation Sequencing (NGS). NGS technology can detect almost all kind of genomic changes that occur in cancer that is different from normal condition. The cost also is now reasonably low to use in routine clinical practice. This chapter will review four kinds of NGS-based biomarkers that are already being used in clinical practice although the routine use is controversial, and that are promising and under active investigation focusing on studies done in Seoul National University Hospital (SNUH).