This document provides guidance on developing thematic reports for a Transboundary Diagnostic Analysis (TDA). Thematic reports are the main source of supporting information for the TDA and are drafted by national experts. The document outlines a two-step process for developing the reports: 1) key areas and national experts are identified based on prioritized transboundary problems, and 2) reports are drafted and reviewed by the TDA development team. Examples of thematic reports from previous TDAs are provided. Advice is given to allow sufficient time for reports, ensure appropriate expertise, and develop clear terms of reference.