Synaptic plasticity is thought to be the neuronal correlate of learning. Moreover, modification of synapses contributes to the activity-dependent homeostatic maintenance of neurons and neural networks. In this chapter, we review theories of synaptic plasticity and show that both homeostatic control of activity and detection of correlations in the presynaptic input can arise from spike-timing dependent plasticity (STDP). Relations to classical rate-based Hebbian learning are discussed.