Diffuse optical imaging is widely used in biomedical research and clinics. Compared with other medical imaging methods, such as magnetic resonance imaging (MRI), X-ray computed tomography (CT), positron emission tomography (PET), and ultrasound imaging, diffuse optical imaging uses diffused light absorbed and scattered by tissues for imaging. This approach is non-invasive and label-free, has a wide field, and quantitatively measures the concentrations of various components such as oxyhemoglobin, deoxyhemoglobin, blood oxygen, water, lipids, and melanin. Furthermore, it collects and assesses tissue functional information. Diffuse optical imaging is advantageous in terms of safety, specificity, and system cost. This article introduces the basic principles of diffuse optical imaging, including the interaction between light and tissue and light propagation models, and summarizes the relevant methods and applications of diffuse optical imaging, including pulse oximetry, diffuse optical spectroscopy, diffuse optical tomography, fluorescence molecular tomography, and spatial frequency domain imaging. Moreover, the prospects for the future development of diffuse optical imaging are presented.