Innovation has become a key factor for economic growth, but how does the process take place at the level of individual firms? This book presents the main results of the OECD Innovation Microdata Project -- the first large-scale effort to exploit firm-level data from innovation surveys across 20 countries in an internationally harmonised way, with a view to addressing common analytical questions. Through the use of common indicators and econometric modeling, this analytical report presents a broad overview of how firms innovate in different countries, highlights some of the limitations of current innovation surveys, and identifies directions for future research.