Cancer remains one of the principal threats to human health. More than 7 million people die of cancer worldwide each year. The high cancer mortality is due mainly to the lack of early detection modalities, especially for internal organs. Computerized tomography (CT), magnetic resonance imaging (MRI), and ultrasound imaging are commonly used diagnostic tools, but they have issues of relatively low resolution, low contrast, radiation risk, or high cost. These imaging modalities typically provide resolutions of 0.1–1 mm. On the other hand, several optical imaging techniques, such as confocal microcopy, nonlinear optical (NLO) microscopy, and optical coherence tomography (OCT), provide much higher resolutions. Confocal and NLO microscopy can achieve submicrometer resolutions while OCT's resolution ranges from 1 to 15 micrometers depending on the light source employed. These high resolutions are achieved by spatial gating of a pinhole for confocal microscopy, spectral gating of harmonic generation or multiphoton absorption for NLO imaging, and coherence gating for OCT. With such cellular or even subcellular resolutions, early cancers can be detected.