Stability of fast pyrolysis bio-oils is a complex phenomenon, especially as numerous compounds with different reactivities are involved. Acid-catalysed condensation and polymerization reactions of carbonyl compounds and reactive lignin-derived radical species take place which increase the share of water-insoluble compounds, average molecular weight and viscosity of bio-oil. Aging can be slowed by dilution, removing of alkali metals catalysing the aging reactions, removing hemicelluloses causing acidic environment, or adding antioxidants to stabilize reactive specimens. In some cases, controlled aging can be used to stabilize bio-oil. Enhanced stabilization can be achieved by esterification and acetalization of carboxylic acids and carbonyls, respectively. Stage-fractionation of bio-oil and separate stabilization of these fractions is one alternative approach. In low temperature catalytic hydrodeoxygenation reactive carbonyl compounds are stabilized by hydrogen. All these stabilization methods have advantages and disadvantages. The appropriate stabilization method will depend on the bio-oil application. When hydrocarbon fuels are the desired product, more expensive hydrotreatment (HDO) routes to stabilization might be justified. When targeted as feedstock for an oil refinery, thermal stability is a key factor. Viscosity increase would be an indicative factor on polymerization tendency of the oil but also oxygen content, distillability, and micro carbon residue.