Positron emission tomography (PET) techniques have made it possible to measure changes in target molecular in living human brain. PET can be used to investigate various brain functions such as receptors, transporters, enzymes and various biochemical pathways; therefore, it could be a powerful tool for molecular imaging of functional neurotransmission. Since dopamine is an important molecule for pathophysiology of Parkinson's disease and schizophrenia, we reviewed in vivo imaging studies focusing on dopaminergic transmission. Dopamine D2 receptor occupancy by antipsychotics and it's time-course have been measured using PET. This approach can provide in vivo pharmacological evidences of antipsychotics and establish the rational therapeutic strategy. PET is a powerful tool not only in the field of brain research but also drug discovery. On the other hand Dopamine D1 receptor is highly expressed in the prefrontal cortex, has been implicated in the control of working memory, seeking, craving, reward. We propose that dysfunction of Dopamine D1 receptor signalling in the prefrontal cortex may contribute to the negative symptoms and cognitive deficits seen in schizophrenia and we suggest that Dopamine D1 receptor binding and cerebral blood flow changes in ventral striatum play the important role of cigarette craving.