Today, many different statistical processes have become an integral part of test development processes in order to examine the qualities that measurement tools should have. With the widespread use of measurement tools, these procedures have been accepted as a "standard" for many researchers. In particular, confirmatory factor analysis and explanatory factor analysis are carried out to examine the construct validity. On the other hand, it is seen that these analyzes carried out today are carried out with techniques based on item response theory. In addition, the examination of structural equivalence, person congruence, and differentiated item functions are increasingly used in studies to determine validity. In addition to providing theoretical explanations on this new understanding of the validity and reliability, this book also includes a content that explains how the analyzes are carried out in the R software environment and how the findings are interpreted. For this reason, the R software environment is introduced in a part of the book. In order to make it easier for researchers to make applications, the R codes of the sample analyzes and the data sets used are also available to the readers. With this book, it is hoped that the frequency of use of modern test development applications in future studies will increase.