Neuronal plasticity is both a basic and one of the most fascinating properties of the adult brain. It is thought to underlie learning processes and our brain's ability to adapt to changes in the external environment. Neuronal plasticity comprises changes in neuronal circuit activity that are provided by modifications of existing or the establishment of novel synaptic connections between neurons. Modifications of synaptic strength or gain and loss of synapses are measurable functionally, for instance directly by probing for changes in synaptic currents passing from one neuron to another or by detecting new or lost electrical connections between them, but also more indirectly for example by monitoring changes in network activity. For example, shifts of the cortical representations of sensory modalities are the result modifications of the synaptic weights between multiple neurons in the network. These functional changes may have structural underpinnings. Strengthening and weakening of existing synapses respectively translate to increases and decreases in the sizes of the structural proxies of synapses, axonal boutons and dendritic spines. Novel connections can be uncovered by searching for the formation or pruning of these same structures. These two concepts have given rise to two facets of plasticity, which can be considered as two sides of the same coin: functional and structural plasticity.