Polar Mesospheric Summer Echoes (PMSE) are unusually strong radar echoes coming from a thin layer at the mesopause (around 88 km), in summer at high latitudes. They were detected for the first time in 1981 with a VHF radar and an explanation for their existence has so far not been forthcoming. In this thesis a unique set of radar echoes jointly taken on the EISCAT UHF and VHP radar systems is presented and analysed. Correlations between the 2 sets of data suggest the systematic occurrence of an electron bite-out layer in the close vicinity of PMSE. On the basis of these observations we propose a model by which electron diffusion would be greatly reduced by the presence of negatively charged particles. This reduced diffusion would then allow the occurrence of turbulent structures at sizes much smaller than is usually allowed in the mesosphere. VHP scattering is enabled by particles of a radius around 10 nm while particles at least 64 times more voluminous are needed for UHF scattering to occur. The occurrence of a bite-out is also explained by an electron scavenging scheme. The electron depletion would be governed by fairly large ice particles which would become negatively charged by this very scavenging scheme and thus would provide the heavy negative particles required for the diffusion reduction. The end result is a cloud model featuring the dynamics and growth of ice particles in polar mesospheric summer conditions under tidal activity. This model shows that conditions for the bite-out, and subsequently PMSE, can only be satisfied sporadically and over a very narrow layer located 3 km above the mesopause. The final conclusions show good agreement with experimental data.