Development generates a dynamic internal environment that is predicted to impact the fitness effects of mutations that arise in populations. Moreover, the external environment in which development occurs can impact the expression of traits within and across life stages. We and others have shown that even fundamental properties of metabolism (e.g., the scaling laws that are thought the govern the relationship between metabolic rate and mass) can vary across development and as a function of the developmental environment (Greenlee, Montooth, Helm. 2014 ICB 54:307). For example, I will show that the scaling of metabolic rate as a function of mass in Drosophila depends on the developmental thermal environment, and that mitochondrial-nuclear genotype can significantly affect larval metabolic rate. This latter genetic effect is itself conditional on the developmental thermal environment, and we have found that interactions between genotype and developmental environment affect metabolic rate plasticity (i.e., the Q10 for metabolic rate) (Hoekstra, Siddiq, Montooth. 2013 Genetics 195: 1129). In addition, I will show that genetic variance in metabolic rate varies across larval development. To better understand the underlying mechanisms that govern the development of energetic processes, we have characterized larval metabolic rate and aspects of mitochondrial physiology across development for a number of natural D. melanogaster genotypes, as well as for mitochondrial-nuclear genotypes that generate energetic inefficiencies. To better understand how metabolic development and plasticity impacts organismal performance, I will relate these patterns to plasticity in development and survivorship in the presence of ethanol, an important ecological challenge and resource for Drosophila larvae.