To observe internal detail in biological structures using light or electron microscopy, specimens need to be prepared from thin sections. Quantitative analysis of these section requires the transfer of complete section images, or selected features from them, into a computer. Where a complete structure is represented by a series of consecutive sections, images may be combined to represent the three dimensional structure. An interactive computer system is described which enables selected features of serial section images to be entered into the computer, edited, reconstructed in three-dimensions and displayed in any orientation. Sections can be analysed individually using appropriate, compatible programs, and an assessment of the complete structure obtained by interpolating between them. Categorisation of the different substructures within the sections allows them to be analysed and displayed separately.