Hydrometeorological investments have trended an overall rise since the mid-1990s when the World Bank’s (WB) investments in National Meteorological and Hydrological Service (NMHSs) were structured as small-scale activities, and have gradually shifted toward developing more complete systems. This guidebook aims to provide a structured basis to approach the task of transforming and developing capacities of national meteorological and hydrological services in the WB client countries. The guidebook’s primary focus is to help the World Bank task teams and development partners working with national governments to strengthen NMHSs’ capabilities in producing and delivering services required by the governments, stakeholders, partners, and users. The guidebook will also serve to explain the process of transformation to NMHSs’ managers and staff. It also intends to help development institutions that are committed to NMHSs’ support through the Alliance for Hydromet Development launched at the Conference of the Parties (COP25) in December 2019.