This book reflects upon the current trends arising in both the theoretical study and the practical modeling of phenomena that exhibit fractionality. There is a tendency to transition from one type of fractionality to several types presented simultaneously; thus, there is a need to construct models that include different types of fractionality. The authors of this book argue that one of the simplest approaches, suitable for applications in engineering, economics and finance, is the modeling of complex fractal properties using mixed models that are linear combinations of elementary fractional processes. With this approach we can adequately model different levels of fractionality and not complicate the model too much. Based on these arguments, this book is devoted to mixed fractional Gaussian processes, that is, the linear combinations of fractional Brownian motions and sub-fractional Brownian motions. Beginning with a detailed description of the properties of separate fractional Gaussian components, this book will be useful both for beginners and for researchers and practitioners.