This volume contains twenty contributions in the area of mathematical physics where Fritz Gesztesy made profound contributions. There are three survey papers in spectral theory, differential equations, and mathematical physics, which highlight, in particular, certain aspects of Gesztesy's work. The remaining seventeen papers contain original research results in diverse areas reflecting his interests. The topics of these papers range from stochastic differential equations; operators on graphs; elliptic partial differential equations; Sturm-Liouville, Jacobi, and CMV operators; semigroups; to inverse problems.