The aim of this book is to find a solution to make large scientific computing in an easy way. To do that, research on Gauss Jordan algorithm is made and a new parallel programming paradigm is presented. The new parallel version can achieve maximum degree parallelism between operations. Also the Gauss Jordan algorithm as an excellent example is used to evaluate different experimental environments and tools. Experiments with YML, OmniRPC and XtremWeb on Grid and Desktop Grid environments testify YML, which is a workflow based programming lanaguage, can be a good solution for end users to make large scale scientific computing for its series of good features such as higher level interface, component reuse and acceptable overhead. According to analysis made in this book and the common features of Clouds possessed, YML-PC, a reference architecture based on workflow for building scientific Private Clouds is proposed. YML-PC inherits those good features YML possessed and some other key technologies such as "data persistence", "available time prediction" and "evaluation on heterogeneous computing nodes". Some evaluations are made on Grids, Desktop Grids and Private Clouds.