Understanding how light can modify chemical reactions has attracted a lot of experimental and theoretical interest, especially after the development of modern photonics. For molecular dynamical processes, introducing external light can partially control the energy flow and chemical reaction pathways. In the first three chapters of this dissertation, we consider the photo-induced coupled nuclear-electronic motion in the presence of a strong laser field. The external monochromatic light induces avoided crossings between ground and excited potential energy surfaces, triggers stimulated absorption and emission and thus alters the evolution pathway of nuclear degrees of freedom. We present a few variants of the famous fewest switches surface hopping formalism (FSSH). We demonstrate some critical modifications to the existing Floquet FSSH algorithm for standard laser-controlled processes: an interference between nuclear wavepacket on different Floquet states and a very robust algorithm to overcome the light-induced trivial crossings between Floquet states. Next, we extend Floquet FSSH to incorporate the Berry phase effects for cases with a non-trivially complex-valued Hamiltonian and present the Floquet phase-space surface hopping (F-PSSH) algorithm. These advances in algorithms allow us to simulate non-adiabatic dynamics in the presence of monochromatic driving light by combining with state-of-the-art electronic structure methods. In the next three chapters, we focus on collective phenomena and enter the electronic strong light-matter coupling regime. First, we discuss the interplay between disorder and collective phenomena such as superradiance and formation of polaritons, and the resultant absorption, transmission and emission spectra using a set of quantum electrodynamics Hamiltonian. Second, we investigate the possibility of using cavity and polaritons to selectively excite molecules inside a gold cavity with a mixed quantum-classical Maxwell-Bloch equation. Third and finally, we attempt to understand the ballistic exciton-polariton energy transport phenomena inside a 2-dimensional cavity. These investigations have impacts for understanding a host of modern experimental observations.