Understanding of the scattering processes due to the interactions between electrons, spin and lattice vibrations is essential to describe some of the most relevant phenomena in condensed matter physics, such as superconductivity, ultrafast magnetism or laser-induced phase transitions. Furthermore, the recent developments on ultrashort laser pulses and time-resolved spectroscopy have made it possible to experimentally monitor the energy and angular momentum transfer between electrons, lattice vibrations and spin waves 1,2 . These experimental observations have shown that the interaction between those quasi-particles are at the core of newly discovered phenomena originated from the non-equilibrium dynamics triggered in a solid after ultrafast laser excitation 3,4,5 . However, and in spite of the recently gained knowledge, the evolution and duration of these transient non-equilibrium states and their microscopic description still remain elusive due to the complex interplay between quasi-particles. Here, we present a novel and general theory that governs the laser-induced out-of-equilibrium system dynamics. Combined with first-principles calculations, it provides a quantitative description of the energy transfer between electron, phonon and magnons and allows the determination of the laser-induced relaxation dynamics. The key features of the model is the use of a non-equilibrium theory of energy conservation along with an explicit dependence of the interaction terms on the phonon and magnon modes and branches 6 . The numerical solution of the model enables to disentangle the interplay between phonons and electrons, phonon and magnons and between phonons and other phonon modes, and to unveil the time dependent system dynamics. The ab initio determination of the electron-phonon, magnon-phonon and phonon-phonon coupling parameters are the building blocks of our fitting-free theory which produces results in excellent agreement with recent experimental observations. References: [1] Y. Hashimoto, R. Iguchi, Y. Oikawa, K. Shen et. Al, Nat. Comms. 8, 15859 (2017) [2] S. Maehrlein , I. Radu, P. Maldonado, A. Paarmann, M. Gensch, et. al, arXiv:1710.02700 [3] T. Henighan et al, Phys. Rev. B 93 220301(R) (2016) [4] D. M. Kennes, E. Y. Wilner, D. R. Reichman and A. J. Millis, Nat. Phys. 13, 479 (2017) [5] A. H. Reid, X. Shen, P. Maldonado, T. Chase, E. Jal, et. al, Nat. Comms. 9, 388 (2018) [6] P. Maldonado, K. Carva, M. Flammer and P. M. Oppeneer, Phys. Rev. B 96, 174439 (2017)