The chapter describes methodologies for harmonized phenological assessments based on a limited set of development phases: flushing, flowering, secondary flushing, color change, and leaf/needle fall. Manual phenological observations are based on a brief examination in the forest stands. More recently, the use of terrestrial digital image photography for forest phenology monitoring has been adopted. Vegetation indices, such as the normalized difference vegetation index (NDVI) have been used for many years to quantify the phenology of different ecosystems. For satellite-based remote sensing of vegetation phenology, phenological metrics are derived from time series of optical data and represent the only possible assessment of phenology over large and inaccessible regions. All indirect methods using optical vegetation indices from digital camera or NDVI sensors need to be validated against ground observations, for which manual tree phenological observations from the forest monitoring plots are often used. Examples from phenological monitoring in Slovenia, France, United Kingdom, and Finland are presented.