Hand in hand with ocean warming, ocean acidification is increasingly threatening life in the world’s oceans. This has sparked a growing number of studies of the effects of ocean acidification and warming on water breathing, ectotherm marine animals throughout the last decade. Amongst those species, fish have generally been regarded as reasonably tolerant towards ocean acidification, due to their well-developed capacities for ion- and pH-regulation. As a result, there are only a handful of physiological studies on ocean acidification effects in fish, and only very few studies have dealt with metabolic regulation at the cellular level. Yet, increased costs of acid-base regulation and metabolic rearrangements may either entail adjustments of the animal’s energy budget, leading to reduced growth and fecundity, or a higher total aerobic energy demand for the whole animal, reducing aerobic scope. In this presentation, I will focus on the current knowledge of the effects of ocean acidification and warming on the cellular metabolism and molecular regulation in various ontogenetic stages of marine fish of different biogeographic origin. I shall try to indicate capacities and bottlenecks for acclimation and discuss the consequences and limitations for the whole animal level with regard to population resilience.