Forstnerič achieved his most notable scientific results tackling problems of complex analysis and geometry: boundary regularity of proper holomorphic maps, polynomial convexity, proper holomorphic maps and embeddings into Euclidean spaces and other complex manifolds, holomorphic automorphisms of Euclidean spaces and their applications, construction of noncritical holomorphic functions on Stein manifolds and Stein spaces, the Oka principle and its application to complex geometry, and nonlinear holomorphic approximation theory. Following a decade of intensive research in the Oka-Grauert-Gromov theory and after solving a problem posed by Mikhail Gromov in 1989, Forstnerič introduced in the literature a new class of Oka manifolds in 2009, and he presented a comprehensive treatment of this subject in his monograph Stein Manifolds and Holomorphic Mappings published by SpringerVerlag in 2011, with the second edition in 2017.