"In this thesis, we study paradigmatic quantum systems coupled to external environments which can be unbounded or bounded. An unbounded external environment can be characterized by few decay channels, where the excitation can be lost. Due to the competition of the states of the system to decay infew channels of the external environment, cooperative eects such as superradiance and subradiance can be induced. Together with the coupling to an unbounded environment, we also consider a bounded external environment. Such coupling will induce a transition to a gapped regime, where the ground state is separated from the other excited states."