The possibility of confinement of photons in a microcavity and excitons in quantum wells opens a way to achievement of the strong light-matter coupling regime with the formation of elementary excitations known as polaritons (exciton polaritons, plasmon polaritons, etc) having hybrid half light half matter nature. This book is devoted to theoretical investigation of several aspects of the exciton-polariton physics, including (i) description of the relaxation dynamics of polaritons in real space and time using the density matrix formalism; (ii) investigation of nonlinear effects in terahertz emission from microvavities; (iii) investigation of the emission spectrum of a quantum dot with broken inversion symmetry embedded in a single-mode microcavity; (iv) investigation of the coupling between a giant plasmonic resonance of a metallic cluster and a cavity mode. The density matrix approach can be applied to two- and one-dimensional systems.Quantum dots and clusters represent zero-dimensional objects. Thus, two-, one- and zero-dimensional objects are considered.