Diaphragms are fundamental parts of the lateral load resisting system (LLRS) of any building, regardless of the material used. They not only transfer horizontal loads to the LLRS but also tie together all structural and non-structural elements, providing integrity to the entire building. The current worldwide trend to build with mass timber has led to the introduction of mass timber in many diaphragm applications. Mass timber diaphragms consist of larger and thicker timber panels made of products such as glued laminated timber (GLT or glulam), cross-laminated timber (CLT), nail-laminated timber (NLT), and dowel-laminated timber (DLT). Despite their fundamental roles in building performance, how mass timber diaphragms and their components perform under in-plane loads remains relatively under-researched. In addition, design guidelines for mass timber diaphragms are not as comprehensive as those for other parts of mass timber buildings in Canadian, U.S., and international building codes and material standards. Consequently, the main objective of this document is to present state-of-the-art information on the performance of mass timber diaphragms under in-plane loads, illuminate the methods used to analyze mass timber diaphragms, and summarize the available information on the design of mass timber diaphragms available in the Canadian, U.S., and international building codes and material standards.