This paper presents recent experimental results and plasma edge stability studies of ELMs, for both H-mode and Optimised Shear (OS) plasmas in JET. The high triangularity (d~0.5) ELMy H-mode experiments in JET are presented in the first part of the paper. These plasmas achieved simultaneously high confinement (H97=1) and high density (~0.9nGR). In this scenario, high pedestal pressure and type I ELMs are observed up to the highest density (~1.1nGR). Moreover, at high pedestal density, an anomalous decrease of type I ELMs frequency and energy losses per ELM is observed, associated to an increased MHD signal between type I ELMs with mainly n=-8 toroidal number, and frequency in the range of <30Hz kHz. This behaviour was associated with type II-like ELMs activity by analogy with ASDEX­U observations. Type II-like ELMs were observed in JET at rather low q95~3, compared to other machines. The coupled JETTO-MISHKA transport and MHD stability analysis for high triangularity, high density scenarios confirmed a possible access to the second stability zone for the ballooning modes and stability for peeling modes at the same time. The second part of the paper is devoted to the experimental comparative study of the transition from Type I to Type III ELMs , carried out in the same plasma configurations for standard ELMy H-modes and OS plasmas. The type III to type I ELMs transition OS discharges is found to deviate strongly from the known power scaling for this transition, derived for standard H-mode plasmas. In fact, type III ELMs are still observed at much higher power in OS scenarios, and this experimental observation suggests that larger current fraction at the plasma edge in OS discharges , compared to standard H-mode could prevent the type III to I ELMs transition. This observation for OS shear plasmas is attributed to peeling modes instability at the edge, suppresses at lower power in standard H-modes.