Symmetry and Symmetry Breaking in Semiconductors : Fine Structure of Exciton States This book discusses group theory investigations of zincblende and wurtzite semiconductors under symmetry breaking conditions. The text presents the group theory elements required to develop a multitude of symmetry breaking problems giving scientists a fast track to bypass the need for recalculating electronic states. The text is not only a valuable resource for speeding up calculations but also illustrates the construction of effective Hamiltonians for a chosen set of electronic states in crystalline semiconductors. Since Hamiltonians have to be invariant under the transformations of the point group the crystal symmetry determines the multiplet structure of these states in the presence of spin orbit crystal field or exchange interactions. Symmetry breaking leads to additional coupling of the states resulting in shifts and or splittings of the multiplets. Such interactions may be intrinsic as in the case of the quasi particle dispersion or extrinsic induced by magnetic electric or strain fields. Using a power expansion of the perturbations these interaction terms can be determined in their parameterized form in a unique way. The hierarchic structure of this invariant development allows to estimate the importance of particular symmetry breaking effects in the Hamiltonian. A number of selected experimental curves are included to illustrate the symmetry based discussions which are especially important in optical spectroscopy. This text is written for graduate students and researchers who want to understand and simulate experimental findings reflecting the fine structure of electronic or excitonic states in crystalline semiconductors Introduction Symmetry Breaking in Spin Degenerate Conduction Bands of Zincblende Type Crystals Symmetry Breaking Effects in Valence Bands of Zincblende Type Crystals Exciton Ground State in Zincblende Type Semiconductors Pseudo Spin Development of the Exciton Ground State in Zincblende Type Semiconductors Invariant Expansion and Electron Band Structure Effects in Wurtzite Type Semiconductors Excitons in Wurtzite Type Semiconductors Light Matter Interaction and Exciton Polaritons in Semiconductors Appendix Index