In this contribution we give a full account of the approach of femtosecond, time-resolved mass spectrometry in molecular beams for the studies of the elementary steps of complex reactions and the application to different systems. The level of complexity varies from diatomics to polyatomics, from direct-mode to complex-mode, from one-center, to two-center, to four-center, and from uni-to bimolecular reactions. The systems studied are iodine, cyanogen iodide, methyl iodide, iodobenzene, 1,2-diiodotetrafluoroethane, mercury iodide, benzene iodine complexes, and methyl iodide dimers. By resolving the femtosecond dynamics and simultaneously observing the evolution of velocity, angular, and state distribution(s) of the reaction, we are able to study multiple reaction paths, the nature of transition-state geometry and dynamics, coherent wave-packet motion, evolution of energy disposal, and the nonconcerted motion in multicenter reactions. These phenomena and concepts are elucidated in dissociation, elimination, and charge-transfer reactions and in the inelastic and reactive pathways of bimolecular reactions. Theoretical phenomenology, using frontier orbitals and molecular dynamics, are invoked to provide a relationship between the observed dynamics and molecular structures.