Stability theory has seen rapid development over the past decade. This textbook gives a modern introduction to the hydrodynamic stability th eory of shear flows. It treats both classical theories such as modal a nalysis, weakly nonlinear theory and resonances, and more recent devel opments such as non-modal analysis, receptivity and secondary instabil ity theory. The last chapter addresses transition in shear flows and a pplies numerous techniques developed in the book to analyze and quanti fy transition scenarios for a variety of shear flows. Simple model pro blems to motivate mathematical concepts are used throughout the book, as are experimental results and computed illustrations to complement t heoretical findings.