In the transition from a fossil-based to a bio-based economy the introduction of bio-based chemicals can be achieved via two distinctly different approaches. The first approach is based on the conversion of bio-mass into existing (petro)chemicals; the ‘drop-in’ approach. The main benefit of this approach is that it can make optimal use of the existing knowledge base and infrastructure. For example, the development of bio-based terephthalic acid will allow for the production of bio-based PET. The second approach is based on the development of bio-based chemicals with a unique structure and functionality. In this approach the biomass is selectively defunctionalized to increase stability and reduce the number of functional groups in order to enhance selectivity, while retaining (part) of the unique structural characteristics of the biomass feedstock. In this chapter we discuss four types of biomass-derived monomers (and derivatives) and their application in polyesters; three building blocks with a more-or-less unique structure (i.e. isosorbide, furan-2,5-dicarboxylic acid and succinic acid), and one true ‘drop-in’ replacement (i.e. bio-terephthalic acid). For each type of building block the chemical structure, raw material source and synthesis is described. Subsequently, synthesis and properties of the polyesters are discussed and, depending on the type of material, current and future commercial applications.