This paper reviews the literature related to the complex chemical composition and multiphase nature of bio-oils and their practical implications. Over time, bio-oil forms separated phases due to purely physical phenomena (phase stability) or chemical composition changes in storage (aging reactions). Bio-oil multiphase behavior and the formation of separated phases are controlled by the complex chemical composition of these oils. Fast pyrolysis oils from woody biomass are typically observed in a single phase. However, feedstocks with high extractives content and/or high ash content commonly produce oils with more than one phase (an aqueous phase, an upper layer, and a decanted heavy oily phase). The first part of this Review focuses on the effects of feedstock composition, particle size, type of pyrolysis reactor, and condensation systems on bio-oil chemical composition and their impact on stable oils production. The second section reviews our current understanding of fresh bio-oil multiphase behavior and ...