Most of white and brown adipocytes, in spite of their well known different functions: i.e. storing energy (white) and thermogenesis (brown), are contained together in visceral and subcutaneous depots (adipose organ) in all mammals including humans. A growing body of evidence suggests that the reason for this anatomical mixture could reside in the fact that adipocytes have peculiar plastic properties allowing them to convert directly each other under appropriate stimuli. Under chronic cold exposure white convert into brown to support the need for thermogenesis and under obesogenic diet brown convert into white to satisfy the need of energy storing. We recently showed that both white and brown adipocytes originate from endothelium of capillaries in adipose tissues. Differentiation imply a transformation of poorly differentiate cells that progressively assume structural and functional properties determining a specific phenotype. Electron microscopy defines specific steps of differentiation for white and brown adipocytes development. White and brown adipogenesis can occur in adipose organ of adult mammals under specific environmental requirements such as obesity (whitening) or cold adaptation (browning). The cytological modifications during whitening and browning of adipose organ are quite different from those of white or brown ontogenesis and suggest a direct conversion of mature adipocytes into a different phenotype (transdifferentiation). Morphometric and BrdU data obtained from the analysis of most depots of adult adipose organ in different strains of mice confirmed that this plasticity is mainly due to transdifferentiation. By lineage tracing and graft experiments we also showed that white adipocytes are able to reversibly transdifferentiate into milk-secreting epithelial cells in mammary glands of pregnant and lactating mice. Thus, transdifferentiation is a plastic phenomenon that can occur physiologically in the adipose organ of adult mammals with important implications for the therapeutic control of widely diffuse pathologies because the brown phenotype of the adipose organ is associated with obesity resistance and drugs inducing the brown phenotype curb obesity and T2 diabetes.