Optical coherence tomography (OCT) can nondestructively obtain the cross-sectional information of samples at micron-level spatial resolution, which is important for ophthalmology and endovascular medicine. The OCT amplitude provides three-dimensional (3D) structural information of a sample (for example, the layered structure of the retina) but is of limited use for obtaining functional information such as tissue specificity, blood flow, and mechanical properties. Functional OCT imaging techniques based on other optical field properties, including phase, polarization state, and wavelength, have also emerged. Among these techniques, Doppler OCT, optical coherence elastography, polarization-sensitive OCT, and visible-light OCT, based on dynamic changes in the light-field amplitude, are robust, uncomplicated, and have achieved high clinical success in label-free blood flow imaging. In addition, dynamic light scattering OCT for 3D blood flow velocity measurement, dynamic OCT with the ability to display label-free tissue/cell specificity, and OCT thermometry for monitoring the temperature field of thermophysical treatments are the frontiers in OCT functional imaging. This paper summarizes the principles and applications of the above technologies, analyzes the remaining technical challenges, and envisions the future development of OCT functional imaging.