ESTREMO: Extreme phySics in the TRansient and Evolving COsmos

We present a mission designed for the study of transient phenomena in the high energy sky, through a wide field X-ray/hard X-ray monitor, and fast (< 1 min) follow up observations with Narrow Field Instrumentation. This is based on an X-ray telescope with an area of 1000 cm2, equipped with high-resolution spectroscopy microcalorimeters and X-ray polarimeter. The performances of the mission on the physics of GRB and their use as cosmological probes are presented and discussed.