An image is a two-dimensional representation of a three-dimensional object. Images obtained by physical or optic sectioning can be used for stereological analysis, while those resulting from projection from the surface of the object or from various levels inside the object can be analyzed only astereologically. Stereology is a system of multidisciplinary methods fpr quantitative analysis of a three-dimensional sections made across them. Relative variables (densities) represent quantities of a given constituent per volume unit, while absolute variables indicate the amount of a given constituent in the whole body studied: from two or more relative and/or absolute variables derived variables can be calculated. The initial phase of planing stereo logical analysis implies the construction of a hierarchical model and definition of reference space for each constituent. Then, from the entire population of fields a sample comprising a certain number of fields is selected to represent the entire population. Anisotropic objects with constituents arranged in a certain preference direction require a special approach. Test systems are instruments for manual stereo logical analysis and before use they should be calibrated to a given objective. The test used should be sufficiently accurate and economical. Stereological analysis is made easier by the use of semi.automatic (optomanual) and automatic equipment. Identifying error sources is of great importance in stereological analyses. Astereology is a system of methods for semiquantitative analysis of images derived from uneven surfaces which are not interpretable in a three-dimensional space. Although quantitative image analysis, stereology and astrology are used mostly in normal and pathologic histology, they are being increasingly introduced into clinical work, especially in radiological diagnosis. They provide a precise quantitative insight into the inner structure of an organism and its parts; they are used for calculating correlations with biochemical and physiological data, and for drawing mathematical and statistical inferences.