Thermosets from renewable sources have been a research focus for the last few decades. Bio-based thermoset resins are considered important candidates for sustainable development since they present the potential to reduce both CO2 footprints and the dependence on petroleum. In order to reduce the ecological impact of a polymer without compromising mechanical and thermal behavior, in some cases, partially bio-based raw materials are accepted. High bio-content resins based on low-toxicity raw materials are the goal for future bio-based thermoset polymers. Free formaldehyde bio-based phenol resins and free bisphenol A–epoxy resins are some examples of this green chemistry concept. The importance of the conversion of biomass into sustainable bio-based polymers is discussed. Chemical pathways developed to make monomers for thermoset polymers from vegetable oils, sugar, and bio-based polyphenols (lignin, tannin, cardanol) will be reviewed.