The book "Theoretical Femtosecond Physics" offers a comprehensive examination of the theoretical foundations of femtosecond physics, an emerging field of research. It addresses the interactions of atoms and molecules with pulsed or continuous laser beams, reaching atomic field strengths. By applying non-perturbative methods and numerical solutions to the time-dependent Schrödinger equation, it provides a deep understanding of various physical phenomena, including the ionization of atoms as well as the ionization and dissociation of molecules. The work not only provides theoretical backgrounds for experiments with strong and short laser pulses but also includes numerous exercises and detailed calculations in the appendices that enhance the understanding of complex matter-laser interactions.