Viral safety is an extremely important issue for the clinical application of cell therapy products. The viral contamination of products can lead to serious adverse events in patients treated with them. Therefore, it is strongly recommended to evaluate the viral safety of products using the most advanced science and technology available at the time, based on scientific rationality. We are also obligated to respond to society’s strong interest in and social demand for public health preservation. Because various types of cell therapy products have been developed, some measures to ensure viral safety for each product are common, and others should be addressed on a case-by-case basis. This chapter outlines the basic approach for viral safety confirmation for cell therapy products and introduces our recent work on viral safety using the next-generation sequencer.