Stereological methods provide efficient and reliable tools for determining quantitative parameters of tissue structure on sections. The principles allow estimation of volumetric ratios, surface areas, surface-to-volume ratios, thicknesses of tissue or cell sheets, and the number of structures. These methods are applied in electron microscopy and involve systematic and statistical error considerations. The internal structure of an organ can only be investigated after tissue destruction. Commonly, fixed and embedded tissues are sectioned to preserve two dimensions while sacrificing the third for resolution. Infinitely thin sections allow for three key features: random cutting of structures, representation of n-dimensional structures as (n-1)-dimensional images, and quantitative occurrence of images on sections corresponding to the quantitative presence of structures in the tissue. These features enable the use of sections for quantitative morphological work. Volumetric analysis uses the Delesse principle, which states that the volume fraction of a component can be estimated by measuring the area fraction of a random section occupied by transections of the component. This principle is applied through point-counting, lineal integration, and planimetry. Surface area measurement involves counting intersections of test lines with surface contours. Surface-to-volume ratios are calculated using point-counting and surface estimation methods. Average thicknesses are estimated using test lines and point-counting. The number of particulate structures is determined by counting transections and using relationships between number of structures and volumetric density. A multipurpose test system combines area, points, and lines for various measurements. It enables the determination of surface-to-volume ratios, average thicknesses, and other parameters. This system is used in both light and electron microscopy. For electron microscopy, a denser arrangement of test points and lines is preferred. Practical applications include determining the relative volumes of nuclei, mitochondria, lysosomes, and cytoplasm in cell types, measuring surface areas of cells and membranes, and estimating the number of specific organelles. These methods are efficient, reliable, and can be applied in both light and electron microscopy. Calibration is essential to ensure accuracy, and statistical analysis is used to test for errors. The methods are supported by various references and have been applied in numerous studies.Stereological methods provide efficient and reliable tools for determining quantitative parameters of tissue structure on sections. The principles allow estimation of volumetric ratios, surface areas, surface-to-volume ratios, thicknesses of tissue or cell sheets, and the number of structures. These methods are applied in electron microscopy and involve systematic and statistical error considerations. The internal structure of an organ can only be investigated after tissue destruction. Commonly, fixed and embedded tissues are sectioned to preserve two dimensions while sacrificing the third for resolution. Infinitely thin sections allow for three key features: random cutting of structures, representation of n-dimensional structures as (n-1)-dimensional images, and quantitative occurrence of images on sections corresponding to the quantitative presence of structures in the tissue. These features enable the use of sections for quantitative morphological work. Volumetric analysis uses the Delesse principle, which states that the volume fraction of a component can be estimated by measuring the area fraction of a random section occupied by transections of the component. This principle is applied through point-counting, lineal integration, and planimetry. Surface area measurement involves counting intersections of test lines with surface contours. Surface-to-volume ratios are calculated using point-counting and surface estimation methods. Average thicknesses are estimated using test lines and point-counting. The number of particulate structures is determined by counting transections and using relationships between number of structures and volumetric density. A multipurpose test system combines area, points, and lines for various measurements. It enables the determination of surface-to-volume ratios, average thicknesses, and other parameters. This system is used in both light and electron microscopy. For electron microscopy, a denser arrangement of test points and lines is preferred. Practical applications include determining the relative volumes of nuclei, mitochondria, lysosomes, and cytoplasm in cell types, measuring surface areas of cells and membranes, and estimating the number of specific organelles. These methods are efficient, reliable, and can be applied in both light and electron microscopy. Calibration is essential to ensure accuracy, and statistical analysis is used to test for errors. The methods are supported by various references and have been applied in numerous studies.