An approach, called discretized environment method,
is used to treat exactly non-Markovian effects in open quantum systems.
In this approach, a complex environment described by a spectral function
is mapped into a finite set of discretized states with an appropriate
coupling to the system of interest. The finite set of system plus
environment degrees of freedom are then explicitly followed in time
leading to a quasi-exact description. The present approach is
anticipated to be particularly accurate in the low temperature and
strongly non-Markovian regime. The discretized environment method is
validated on a two-level system (qubit) coupled to a bosonic or
fermionic heat-bath. A perfect agreement with the quantum Langevin
approach is found. Further illustrations are made on a three-level
system (qutrit) coupled to a bosonic heat-bath. Emerging processes due to strong memory effects are discussed.