Forest biomass plays a key role in our climate and constitutes a major component of the Earth carbon cycle. However, forests are facing increasingly challenging environmental conditions induced by climate change globally. In this context, monitoring the temporal evolution of the biomass on a global scale is fundamental. Ground-sampling trees regularly in global-scale is very difficult, not only because of the massive resource and time it requires, but also because of the inaccessibility of most forested areas. Space borne instruments can overcome some of these difficulties and are able to probe most of the Earth regularly. Each type of mission comes with its advantages and drawbacks. Optical instruments can map the land surfaces in great details but are heavily affected by atmospheric (clouds, haze, aerosols) and light conditions. Radars and passive microwave radiometers data often has relatively lower spatial-resolution, but they come with all-weather capabilities so their temporal-resolution is very high. In addition, their signal is able to penetrate the vegetation layer so they have particular advantages in soil moisture and biomass monitoring. To combine the strength of both optical and microwave instruments, time series of various indices, derived from both sensors exist and characterize different components of trees (water content, leaf, ...). To further the study of forest monitoring, ESA is planning to launch BIOMASS mission in 2024. It is the first P-band (435 MHz) radar in space that is dedicated to estimating forest biomass. The ESA OSMOSE project therefore aims to produce harmonized optical and passive microwave data cubes spanning more than 20 years. It also aims to create innovative algorithms to reprocess the existing indices and output a dataset dedicated to vegetation monitoring. By using complementary information from optical indices (LAI, FAPAR, NDVI) and biomass estimates derived from the passive microwave radiometers’ observations by the AMSRE/2 and SMOS missions, OSMOSE aims to provide a comprehensive dataset to apprehend in details the global evolution of forest biomass. It includes the development of new methodologies to derive the vegetation optical depth (VOD) which will be further processed to derive forest biomass. In parallel OSMOSE is also developing a method to estimate forest biomass directly from passive microwave brightness temperatures as well as canopy water content from optical sensors. After one year of study, we will present the OSMOSE project and the first associated initial results (e.g. new product datacubes). The next step will consist in evaluating the syngery of all these products with the ultimate goal of closely monitoring the status and evolution of forest biomass over the period 2002 - present.