Label-free optical imaging technology can perform long-term, noninvasive, high-resolution imaging on living cells owing to its noninvasive characteristics. This technology has promising applications in biomedical research and clinical diagnosis. Label-free imaging technologies can be categorized into specific imaging and nonspecific imaging. In this paper, we review the commonly used label-free imaging. Herein, the imaging principles, advantages and disadvantages, as well as the recent progresses of unmarked imaging are introduced in detail. Furthermore, the future development of the label-free imaging technology is prospected.