The monograph systematizes the basic concepts and studies of functions that objectively assess the quality of digital images. The distribution of local image quality scores is shown to be inconsistent with the normal distribution of data, but their arithmetic mean is often used as a generalized score. Described is a new method for quantitative assessment of image quality, using not the average, but Weibull distribution parameters for local estimates. The advantages of the method are presented when comparing images with standards and in their absence. Current topics related to evaluation of digital images of different types at the stage of their registration, when there are no reference images, are considered. Factors that reduce the quality of digital images have been investigated. Fields of application of image quality assessment functions, methods of data normalization, classes of digital images and their possible distortions, available image bases for conducting experiments on comparison of image quality assessments with visual assessments of experts and other topics are described.