Using the principles of the ETH - Approach to Quantum Mechanics we study fluorescence and the phenomenon of ``quantum jumps'' in idealized models of atoms coupled to the quantized electromagnetic field. In a limiting regime where the orbital motion of the atoms is neglected and the velocity of light tends to infinity we derive explicit non-linear stochastic differential equations describing the effective time evolution of states of individual atoms. These equations give rise to a measure on state-trajectories with quantum jumps which is a quantum-mechanical analogue of the Wiener measure of Brownian motion. Our results amount to a derivation of the fundamental randomness in the quantum-mechanical description of microscopic systems from basic principles in the context of some simple models.
