This book gives a status report on where we stand today in studies of cometary nuclei and their immediate environment. The papers were derived from a Horizon 2020 project called MiARD (Multi-instrument Analysis of Rosetta Data), which was designed to use multiple datasets from the Rosetta mission to comet 67P/Churyumov-Gerasimenko to place further constraints on the properties of the nucleus of the comet and its immediate environment. The individual chapters are the result of a significant effort by leading researchers to establish clear statements on what we know from current cometary data. Based on a workshop held in 2018 at the International Space Sciences Institute, the book focuses on the nucleus itself and the relationship between the surface properties and the observed outgassing. Additionally, it makes a first critical assessment of the Rosetta dataset, establishing what would be an appropriate next step in cometary research. Previously published in Space Science Reviews in the Topical Collection "Comets: Post 67P / Churyumov-Gerasimenko Perspectives"