Covalent Organic Frameworks (COF) have recently emerged as a promising class of sustainable photocatalysts. They are made from purely organic building blocks that can be adjusted to produce tailor-made photo-active materials. Understanding of how the light-matter interactions in COFs influence their photocatalytic performance and how we can tune the chemical synthesis to optimize these photophysical properties, is vital for the development and full exploitation of the next generation of sustainable, versatile, and cost-effective COF photocatalysts. By this knowledge we will tune COFs towards two highly relevant photocatalytic applications: (1) production of H2O2 from air and water and (2) oxidation of hydrocarbons by in situ generated radical oxygen species. We aim to understand the entire photocycle in ensembles of COF materials, from optical excitation to the stages of charge carrier formation and transfer, and their subsequent roles in photocatalytic reactions. To achieve this, we will employ time-resolved spectroscopic approaches on the macro-scale to study transient species (excited charges, adsorbed chemicals, and the COF structure itself), essential for capturing and understanding the dynamic range of photo-induced processes. We argue that a better understanding of these light-matter interactions in COFs would allow us to develop tailor-made COF-based photocatalysts..