Recent experimental results and plasma edge stability studies of ELMs are presented for both H-mode and optimized shear (OS) plasmas in JET. High triangularity (δ~0.5) ELMy \mbox{H-mode} experiments in JET achieved simultaneously high confinement (HIPB(y,2) = 1) and high density (~0.9 nGR). At high pedestal density (~1.1nGR), an anomalous decrease of type I ELMs frequency and energy losses per ELM was observed. A specific for high triangularity and high density (δ~0.5 and ne~1.1nGR) MHD activity with broad-band frequency spectrum (