Aldol reactions are important carbon-carbon bond-forming processes crucial to bulk and fine chemical synthesis, with growing interest in their use for converting bio-derived feedstocks into chemicals and fuels. Despite their versatility and wide-ranging applications, controlling selectivity in aldol reactions remains a significant challenge because of the formation of multiple products, including primary self- and cross-condensation derivatives and secondary aldol condensation products. Moreover, the fission reaction, a competing carbon-carbon bond-cleavage event to olefins and carboxylic acids, has garnered interest as a sustainable route for producing industrially relevant compounds such as isobutene from acetone self-condensation. Conventional strategies for selectivity control often rely on soluble catalysts and specialized reagents; however, these methods often involve expensive separations, generate significant wastes, and pose environmental concerns. This dissertation investigates the factors governing selectivity in acid-catalyzed aldol reactions, aiming to develop sustainable, selective pathways for the aldol chemistry. Key reaction pathways in aldol reactions—dehydration and fission—are revealed to proceed as parallel reactions of a shared aldol intermediate. The catalyst type exerts a strong influence on selectivity. While soluble Brønsted acids favor the dehydration pathway, solid Brønsted acids preferentially promote the fission pathway. Poisoning experiments with base probes of varying pore accessibilities underscore the role of void size and Brønsted acid site in driving fission, with larger void sizes in microporous solids correlating with enhanced fission selectivity. The role of acid strength in influencing reaction rates and selectivity is also elucidated using zeolites with varying trivalent heteroatoms. For soluble acids, stronger acid sites accelerate condensation reactions without altering product distribution. For microporous solid acids, both pore topology and Brønsted acid strength influence selectivity and reaction rates toward the fission pathway. Reactant electronic properties are also shown to significantly impact selectivity. Hammett analysis reveals that electron-donating substituents enhance reaction rates, with solid acids exhibiting greater sensitivity to electronic effects than their soluble counterparts. Furthermore, cross-aldol reactions using diverse substrates produce olefins and carboxylic acids of industrial relevance through the fission pathway, although at varying yields. Overall, this work demonstrates that selectivity in aldol reactions can be strategically modulated through careful selection of catalysts and substrates. These findings provide a foundation for advancing selective and sustainable aldol chemistries, offering significant potential for industrial applications.