Optical reporter genes can be exquisitely addressed to sub-cellular organelles through targeted genetic encoding. A protein emitting light upon abrupt rises of local calcium concentrations is genetically encoded into the mitochondria of a mouse. Providing that the sampling of data acquisition is fast enough, the localistion of the signal is attributed to the mitochondria, roughly one micrometre in size each, even though the detection sensors have a millimetric spatial resolution. In neuroscience research, it is a classical paradigm to superimpose molecular images from PET with anatomical images from MRI. New experimental techniques allow co-registering multimodal images with widely differing spatial resolutions, by rebuilding 3D sets of in vivo PET images of the rat brain with post mortem histological stains of the same brains. Extension of such techniques beyond the artisanal knowhow is bound to become a major experimental tool for translation of molecular information into in vivo clinical applications. Learning Objectives: 1. To understand the temporal and spatial scales within living animal systems. 2. To become familiar with the temporal and spatial scales imaged by in vivo and in vitro imaging methods. 3. To learn how to bridge temporal and spatial scales between the different imaging methods.