Extremely metal-poor stars: Tracing the early stages of nucleosynthesis and chemical evolution

Observational knowledge of the origin, history, and distribution of the chemical elements rests upon abundance analyses of interstellar gas and stars of various populations. When looking in detail at the abundance patterns of the most metal-deficient stars of the Milky Way, we are looking at the fossil record of nucleosynthesis processes that took place in the first generation(s) of stars. Each elemental abundance is a powerful diagnostic: from the very light elements that test homogeneous vs inhomogeneous BBN scenarios to the very heavy nuclei that test and constrain the signature of individual supernova (SN) events. Here, the remarkable (observational) progress recently achieved in the field of extremely metal-poor stars will be presented and critically reviewed.