1. Synaptic plasticity is thought to underlie learning and memory formation in the brain. However, how synaptic plasticity is induced during these processes remains controversial. An attractive candidate mechanism for learning at the neuronal level is spike timing-dependent synaptic plasticity (STDP), which depends on the precise (msec) timing of the synaptic input and the post-synaptic action potential. This temporal relationship resembles typical features of associative learning. Here, we review recent evidence suggesting that STDP is likely to underlie certain forms of learning. 2. First, we discuss the cellular mechanisms of STDP elucidated by in vitro experiments. A special focus is put onto aspects known to differ between in vitro preparations and the in vivo situation. 3. Second, we review the experimental induction of STDP in vivo, in various systems ranging from Xenopus tectum to human motor cortex. 4. The last part of the review addresses the question whether STDP can be induced by activity patterns occurring during normal behaviour. 5. We conclude that STDP is a robust phenomenon in vivo and a likely mechanism underlying sensory map plasticity in the neocortex. Further experimental evidence is required to determine whether STDP also has a role in more complex forms of learning.