The development of bark structure of Quercus robur L., Ulmus glabra Huds., Populus tremula L. and Betula pendula Roth is being described. Profound structural changes can be observed during the first years after secondary growth has started. In all four species the epidermis is replaced by a periderm, the cortex shows intensive dilatation growth, and the groups of primary bark fibres are pushed apart. The collapse of sieve tube members starts with the second year. With proceeding secondary growth, the specific formation of sclerenchymatic tissue, especially sclereids, and the dilatation growth are processes which strongly affect the bark structure of Quercus robur, Populus tremula and Betula pendula. In addition, wide, fused phloem rays develop in Quercus robur. The structure of Ulmus glabra bark is affected by the formation of phloem fibre-/sclereid-like cells and mucilage cells and by dilatation growth. The histological pattern of Ulmus glabra bark stabilises to a great extent after the first few years, the other barks investigated show further developmental processes over many years. In all species the formation of a rhytidome is the last distinct modification of bark structure.