This monograph extends the classical spectral theory of ordinary graphs to the broader framework of signed graphs. It integrates foundational results with recent advances, explores applications, and clarifies connections with related mathematical structures while indicating promising directions for future research. The exposition remains rigorous throughout, presenting core concepts, major developments, and emerging ideas in a coherent and accessible manner. Complementing the theoretical material, the monograph includes illustrative examples and problem sections to support understanding and encourage continued study. This monograph will serve as a reference for mathematicians working in the spectral theory of signed graphs as well as a tutorial for graduate students entering the subject area and computer scientists, chemists, physicists, biologists, electrical engineers and others whose work involves graph-based modelling.
