The first chapter lists the basic results of orthogonal polynomials, Jacobi, Laguerre, and Hermite polynomials, and collects some frequently used theorems and formulas. As a base and useful tool, the representation and quantitative theory of Hermite interpolation is the subject of Chapter 2. The theory of power orthogonal polynomials begins in Chapter 3: existence, uniqueness, Characterisations, properties of zeros, and continuity with respect to the measure and the indices are all considered. Chapter 4 deals with Gaussian quadrature formulas and their convergence. Chapter 5 is devoted to the theory of Christo (R)el type functions, which are related to Gaussian quadrature formulas and is one of the important contents of power orthogonal polynomials. The explicit representation of power orthogonal polynomials is an interesting problem and is discussed in Chapter 6. Chapter 7 is a detailed treatment of zeros in power orthogonal polynomials. Chapter 8 is devoted to bounds and inequalities of power orthogonal polynomials. In Chapters 9 and 10 we study asymptotics of general polynomials and power orthogonal polynomials, respectively. In Chapter 11 we discuss convergence of power orthogonal series, Lagrange and Hermite interpolation, and two positive operators constructed by power orthogonal polynomials. In Chapter 12 we investigate Gaussian quadrature formulas for extended Chebyshev spaces. In Chapter 13 we give construction methods for power orthogonal polynomials and Gaussian quadrature formulas; we also provide numerical results and numerical tables.