Pellet Enhanced Performance (PEP) has been observed in a number of JET discharges at various plasma conditions, in both L- and H-modes, with the H-multiplier (the confinement enhancement factor over the Goldston confinement time) covering the range 1 to 4, and with plasma currents from 1MA to 4.1MA . Most of the PEP plasmas have been created by refuelling with pellets of 4mm diameter injected at 1.2km/s. The limitations in the PEP performance seem to be set by at least two mechanisms: impurity behaviour, MHD activity or a combination of both. PEPs show an improved central confinement with an effective heat conductivity reduced by factors of ~2-5 relative to otherwise comparable discharges. This is possibly related to the inverted shear in the plasma core due to the large local bootstrap current density. The stability of the ballooning modes has been studied and the PEP plasma core is found to be in the 2nd stability region against ballooning modes or close to marginal stability. In a number of discharges complex high (m,n) modes have been observed with the soft-X-ray cameras. The behaviour of the low (m,n) MHD modes can be understood only by considering the detailed evolution of the inverted q-profile, which exists in a given discharge.