Main combinatorial results are obtained in the field of graph theory and, in particular, in the theory of graph spectra. This line of research exists in Serbia (Belgrade, Kragujevac, Novi Sad) for more than twenty five years. Results have been published both in domestic and best international scientific journals in the field as well as in a few internationally recognized scientific monographs. Recent results include the study of eigenspaces of the adjacency matrix of a graph and, in particular, graph angles, star partitions and canonical star bases of graphs. Other topics of interest include graph algorithms, combinatorial optimization and applications of graph theory to chemistry. Investigations are supported by the programming package "Graph", developed by our group in early eighties, but still in use and suitable for posing, verifying and disproving conjectures in graph theory.