A review of computer aided diagnostic principles is presented. A brief treatise on the definition and theory of the utilization of machines to aid human diagnostic reasoning forms a foundation for its practical use in current veterinary practice. Understanding a tool is the key to its proper use. Therefore, the intentions of the currently available software are discussed as well as the limitations. Finally, in light of the refined understanding of the use of computers to aid diagnosis, the scope of appropriate use is outlined as an encouragement to make the most of the tool.