Data mining is considered as the analysis step of the Knowledge Discovery in Databases process where it composed of the pre-processing, machine learning and visualization of the input data. Pre-processing stage contain techniques for fixing the pre-assumptions made by some machine learning techniques on input data sets. Medical data sets as a real life data could not follow pre-assumption. Some techniques in the pre-processing stage could lead to a distortion in the internal structure of the medical data. This book disuses this problem and the ways of avoiding it through a set of proposed techniques.