Abstract:  Our Universe was filled with neutral hydrogen and helium after a recomination era that followed the big bang. In this epoch the Universe was dark, cold and opaque. However, it underwent a phase transition,  where the neutral gas was re-ionised, when the first stars began to shine and the first quasars emerge. The Universe became transparent to optical light again, allowing starlight to travel through large distances and reach us today. The reionisation era is one of the least understood periods in the cosmological history. It is where galaxies, like our Milky Way, were in the infant stages, rapidly turning the gases trapped within into luminous stars. In this talk I will discuss the ionisation proecesses in protogalaxies and their host enviroments. The ionisation was generally believed to ve caused by irradiation of UV/X-rays from the massive stars and accreting sources. However, the bombardment of ultra-high-energy particles generated in the violent processes that are associated with star formation and accretion could also play an important role. I will also show how the ionisation fronts form, develop and propagate in the presence of radiative heating and high-energy particle bombardments. The implications on structual formation in the young Univserse will be discussed.