Photochemical reactions range from photosynthesis to atmospheric reactions, and technologies such as sensors or displays. Due to their complexity, they remain the least understood type of chemical process. However, new experimental techniques capable of monitoring photochemical processes in unprecedented detail are appearing. This Faraday Discussion brings together experimentalists and theoreticians working from different perspectives in the field. It provides the opportunity to identify how new techniques can complement each other, to address contention and controversy, and to propose future research.