This Algorithm Theoretical Basis Document (ATBD) describes in detail the methodology used to produce the third collection of the 100m Land Cover product, the global annual maps for the period 2015-2019, that was released under the global component of the Copernicus Land Monitoring Service. The methodology covers the retrieval and pre-processing of Earth Observation satellite (PROBA-V) observations, the calculation of metrics, the main classification, the regression for the versatile cover fraction layers, the time series break detection and so on. Next to the detailed ATBD, a summary Product User Manual describes the product (methodology, quality, contents and format) and Validation Report(s) provide more details on the validation and quality of the product.