Mapping the strong interaction between Rydberg excitations in ultra-cold atomic ensembles onto single photons enables the realization of optical nonlinearities which can modify light on the level of individual photons. This novel approach forms the basis of a growing Rydberg quantum optics toolbox, which already contains photonic logic building-blocks such as single-photon sources, switches, transistors, and conditional π-phase shifts. This thesis reports on two experiments investigating strong photon-photon interactions mediated by Rydberg interactions. First, applying a ladder-type EIT scheme to Rydberg atoms in D-states with high principal quantum number results in the Rydberg interaction mediated nonlinearity accompanied by a time-dependent decay of transmission of probe light through the medium. In a joint experimental and theoretical analysis, this effect is attributed to the dephasing of propagating polaritons into stationary Rydberg excitations caused by the state mixing interaction occurring with RydbergD-state atoms. Second, via a two-photon Raman excitation scheme Rydberg atoms are efficiently excited in a small atomic cloud. As the size of the Rydberg blockade exceeds the dimension of the medium, only a single Rydberg excitation can be present at a time. By fast engineered dephasing this Rydberg excitation is decoupled from the light field. Measurement of the transmitted light and the amount of excited Rydberg atoms gives evidence for the realization of a deterministic single-photon absorber.