This chapter reviews the fundamentals of power system harmonics, discussing their sources, consequences and mitigation methodologies. It presents an in-depth examination of harmonic standards and measurement techniques essential for maintaining power quality. The chapter progresses to detail the modelling of key power system components for harmonic studies, including overhead lines, cables, transformers and generation units, both conventional and converter based. The importance of accurate modelling in understanding harmonic impacts and devising mitigation strategies is emphasized. The final section introduces the concept of probabilistic modelling, highlighting the importance of incorporating uncertainties in system operations, such as variable renewable outputs and fluctuating loads, and the uncertainties in harmonic injections from non-linear sources.