The aim of this book is to honor the memory of Professor Jos Carlos Petronilho and hence focuses on his main research areas (Special Functions, Orthogonal Polynomials, Approximation Theory). It is a collaborative book and among the contributing authors are outstanding leaders in the field. The book addresses different topics exploring the connection between the areas already mentioned and their applications, from different perspectives and using several tools, both analytical and algebraic. Beside the researches working in these topics, the book potentially interests the readers working in areas of Mathematics, Science and Technology where Approximation Theory, Special Functions and Orthogonality are potentially useful tools.