Elevated temperatures and hypoxia often co-occur in the environment, affecting the metabolism of fishes by causing mismatches between oxygen supply and demand. Short-term warming increases metabolic rates, driving the demand for oxygen, whilst acute hypoxia limits oxygen supply. These effects are exacerbated in early-life stages, as embryos are often unable to behaviourally regulate their environment, instead relying on a still developing cardiovascular system to meet metabolic demands. Acute exposure to environmental stress during development is associated with both short- and long-term physiological consequences. Developmental programming comprises long-term changes to an organism's phenotype that are elicited by the conditions experienced during development. Exposure to single stressors during embryogenesis can have wide ranging effects on the later-life physiology of fishes. However, few studies have investigated how co-occurring embryonic stressors may interact to influence an individual's phenotype in later-life, and how these effects vary with age and across species. The aim of this thesis was to determine the effects of elevated temperature, hypoxia, and the combination of the two, on the physiology of fishes throughout ontogeny and examine the stability of the resulting 'programmed' phenotypes. Chapters three, four, and six demonstrate that the environmental stressors used in this thesis (+5 ºC and 50% air saturation of oxygen) are sufficient to cause short-term effects on both species investigated (Scyliorhinus canicula and Danio rerio), eliciting changes in growth, metabolism, and cardiac gene expression. Following these observations, Chapters five and six determine the long-term effects of the embryonic exposures and assess their consistency across species and time points. Developmental warming caused lasting changes to the gene-network organisation of S. canicula, a finding which was recapitulated in three teleost species. These changes in gene network organisation were associated with increased transcriptomic sensitivity to future thermal challenges. Embryonic warming, hypoxia, and their combination caused long-term changes in the growth trajectories of zebrafish, characterised by a leftward shift of the growth curve. These differences in growth trajectory were accompanied by increases in aerobic scope, as well as stressor- and age-specific effects on environmental tolerances. Together, Chapters five and six demonstrate the complex, temporally dependent characteristics of developmental programming, highlighting the need for future multi-stressor, multi-time-point studies in fishes. Finally, Chapter seven investigates the oxygen- and capacity- limited thermal tolerance (OCLTT) hypothesis, a purported framework for integrating the effects of warming and hypoxia. By combining behavioural and physiological data, Chapter seven tests and subsequently provides evidence against a prediction that emerges from OCLTT, supporting data from the previous chapters in suggesting that exposure to warming and hypoxia have distinct, as well as similar, consequences for the physiology of fishes. Climate change is a major threat facing aquatic ecosystems, with exposure during sensitive life-stages capable of affecting a fish's physiology throughout their life. This thesis demonstrates that exposure to warming and/or hypoxia during embryogenesis has both short- and long-term consequences for the physiology and cardiac transcriptomics of fishes. Furthermore, it shows that both later-life physiology and gene expression are influenced by interactions between developmental temperature, hypoxia, and age, highlighting the need for future studies to integrate ecophysiological questions across a range of stressors and life-stages.