The first billion years according to JWST

With stunning clarity, the JWST has revealed the Universe’s first billion years. The scientific community is analysing a wealth of JWST imaging and spectroscopic data from that era, and is in the process of rewriting the astronomy textbooks. Here, as a result of the 2024 ISSI Breakthrough Workshop, we provide a snapshot of the great progress made towards understanding the initial chapters of our cosmic history 1.5 years into the JWST science mission. We present the current census of early galaxies, their luminosities, appearance, chemical composition, masses and formation histories as revealed by JWST. We relate the discovery of massive black holes in early galaxies and discuss their demographics and implications for their formations and growth. We conclude by describing the potential sources of reionization and our current understanding of how the Universe became fully ionized. Throughout the Perspective, we highlight discoveries and breakthroughs, topics and issues that are not yet understood, and questions that will be addressed in the coming years, as JWST continues its revolutionary observations of the early Universe. The infrared view of JWST has opened up the early Universe to study. This Perspective summarizes the key developments in our understanding of the high-redshift Universe from the first 18 months of JWST observations.