Laser Speckle Contrast Imaging (LSCI) is a powerful tool for full-field imaging of blood flow, particularly in the brain, retina, and skin. The technique measures the intensity fluctuations in the speckle pattern of scattered light, which are caused by the Doppler effect when particles scatter light. The theoretical basis for analyzing speckle intensity fluctuations dates back to the 1960s with the development of dynamic light scattering. LSCI has been advanced in the 1990s with faster digital acquisition and processing technologies, and its adoption has increased significantly in recent years, driven by its rapid application in blood flow studies in the brain. The paper reviews the underlying physics of speckle contrast imaging, including the theoretical background of speckle basics, spatial and temporal speckle contrast, and the impact of speckle size and sampling windows on the statistics of laser speckle. It also discusses recent developments to improve the quantitative accuracy of blood flow measures, such as calibration procedures for handling static scattering and more efficient algorithms for calculating speckle contrast. Applications of LSCI in neuroscience, dermatology, and ophthalmology are reviewed, highlighting its use in monitoring skin perfusion, visualizing and quantifying retinal blood flow, and studying functional brain activation, cortical spreading depression, and stroke. LSCI's ability to provide high-resolution, full-field imaging of blood flow dynamics makes it a valuable tool for various biomedical research and clinical applications.Laser Speckle Contrast Imaging (LSCI) is a powerful tool for full-field imaging of blood flow, particularly in the brain, retina, and skin. The technique measures the intensity fluctuations in the speckle pattern of scattered light, which are caused by the Doppler effect when particles scatter light. The theoretical basis for analyzing speckle intensity fluctuations dates back to the 1960s with the development of dynamic light scattering. LSCI has been advanced in the 1990s with faster digital acquisition and processing technologies, and its adoption has increased significantly in recent years, driven by its rapid application in blood flow studies in the brain. The paper reviews the underlying physics of speckle contrast imaging, including the theoretical background of speckle basics, spatial and temporal speckle contrast, and the impact of speckle size and sampling windows on the statistics of laser speckle. It also discusses recent developments to improve the quantitative accuracy of blood flow measures, such as calibration procedures for handling static scattering and more efficient algorithms for calculating speckle contrast. Applications of LSCI in neuroscience, dermatology, and ophthalmology are reviewed, highlighting its use in monitoring skin perfusion, visualizing and quantifying retinal blood flow, and studying functional brain activation, cortical spreading depression, and stroke. LSCI's ability to provide high-resolution, full-field imaging of blood flow dynamics makes it a valuable tool for various biomedical research and clinical applications.