The identification, more than 50 years ago, of mitochondria as the site of oxidative energy metabolism has prompted studies that have unraveled the complexity of the numerous biosynthetic and degradative reactions, fundamental to cell function, carried out by these organelles. These activities depend on a distinctive mitochondrial structure, with different enzymes and reactions localized in discrete membranes and aqueous compartments. The characteristic mitochondrial structural organization is the product of both synthesis of macromolecules within the mitochondria and the import of proteins and lipids synthesized outside the organelle. Synthesis and import of mitochondrial components are required for mitochondrial proliferation, but rather than producing new organelles, these processes may facilitate the growth of pre-existing mitochondria. Recent evidence indicates that these events are regulated in a complex way by several agonists and environmental conditions, through activation of specific transcription factors and signaling pathways. Some of these are now being elucidated. Generation of nitric oxide (NO) appears to be a novel player in this scenario, possibly acting as a unifying molecular switch to trigger the whole mitochondriogenic process.