New diagnostic measurements have driven important steps forward of the JET experimental programme in areas related with H-mode and Advanced Tokamak (AT) physics. As a consequence a programme of diagnostics developments is started. The paper describes the recent new diagnostic measurements and the programme of diagnostics developments on JET. The measurement of the current profile made using MSE(motional stark effect) during strongly reversed shear discharges has shown a large region of low current at centre; the first preliminary measurement of poloidal velocity (made using the CXRS) shows some agreement with the neoclassical calculations; the measurement of the edge density profile made using Li-beam diagnostic and edge LIDAR Thomson Scattering has contributed to ELM physics study; the real time control of the internal transport barriers has improved the steady state capability of AT discharges; a measurement of fast He beam spectroscopy has been carried out, possibly leading to the determination of plasma parameters (Te and Ne) from edge to the pedestal. The diagnostics development programme will give the possibility of contributing to a deeper understanding of i) the link of current profile and the transport properties of the discharges; ii) the pedestal behaviour and the MHD stability (ELMs) properties of the H-mode; iii) fast ion driven MHD; iv) further study and control of tearing modes; v) DT specific themes including the isotopic dependence of the confinement; vi) ITER specific technology to be tested; vii) the possibility of realizing a steady state AT discharge.