AbstractResource assessment is a necessary step for any project, plan, or future energy prospectus involving renewable energy sources. The assessment of biomass and, in particular, the so-called forest and agricultural field residues, faces particular methodological difficulties due to the scarcity and heterogeneity of the data sources. For agricultural residues such as cereal straw, bagasses, etc., the residue to product ratios (RPR) are the key data needed for the estimations. In the present work the values of these product ratios reported in the literature are surveyed and are seen to vary greatly, depending on the reporting source. Some methodological procedures for obtaining RPR values are considered, and guidelines for conducting the resource evaluation are indicated. For the estimation of forest field biomass resources a methodological procedure based on the different stand stages along a forest rotation is presented. The main steps of this methodology are based on the availability of basic quantitative data from forest stands and the assumption of different silvicultural operations during the stand rotations. Environmental constraints should be observed in biomass resource assessments. However, the lack of clear recommendations concerning biomass removal in different forest soil and climate conditions suggest that more research is required to assess the sustainability of biomass harvest. Chemical characterization of some of the most representative biomass materials is also presented.