The chapter will summarize and critically review metabolic responses to environmental stress and stress effects on energy budgets in mollusks. Metabolic adaptations to environmental stress will be analyzed including alternative energy conserving pathways, aerobic/anaerobic transitions and metabolic arrest. Their role in the efficient use of energy substrates and in the maintenance of cellular homeostasis and redox status will be discussed. General stress effects on energy budget at different organization levels (from the cell to the whole-organism) will be analyzed including energy uptake, assimilation, conversion and expenditure, as well as the metabolic costs associated with stress resistance (including antioxidant protection, chaperone function and damage repair). We will compare metabolic responses to moderate and extreme levels of environmental stressors using temperature, ocean acidification, salinity, oxygen deficiency and toxic metal stress as examples. This chapter will also outline the future research directions to improve our understanding of the stress effects on bioenergetics in mollusks and other invertebrates. Keywords: Stress, metabolic adaptation, metabolic arrest, bioenergetics, cellular defense, energy trade-offs