Last decade there was a remarkable progress in the field of exciton-polaritons which occur inside a high quality semiconductor microcavity due to the strong light-mater coupling. Thanks to extremely small effective masses these composite bosons provide a promising platform for the investigation of the spontaneous formation of coherence in periodic lattices. For instance, spontaneous build-up of both in-phase and anti-phase “super-fluid” states as well as condensation in localized gap soliton states have been observed. This presentation shall provide a review of the nonlinear dynamics of exciton-polaritons in incoherently pumped semiconductor microcavities with embedded periodic lattices. Different regimes of relaxations and oscillatory dynamics are governed by superposition of Bloch eigenstates in such systems. This paves the way for a controlled loading of polaritons into a particular energy state of a band-gap structure enforced by the periodic potential.