Positron emission tomography (PET) is a useful technique to quantify various target molecules in vivo such as neuroreceptors, transporters and amyloid plaques using various successful radioligands. The technique has been widely used for the drug development in recent years. There are several approaches such as microdosing, measurement of in vivo receptor occupancy, and biomarkers. As for microdosing, the biodistrubution of the drugs in the human body could be evaluated in the very early phase of the drug development. The measurement of receptor occupancy in vivo could help to determine the optimal doses clinically before the largescale clinical trials are perfomred. As biomarkers, radioligands could detect the pathological changes such as the accumulation of betaamyloid and microglia. Especially the measurement of dopamine D2 receptor occupancy has been useful in the evaluation of antipsychotics. Based on it, the optimal clinical doses were evaluated and determined for the patients with schizophrenia. Although currently available antipsychotics have efficacy to the positive symptoms such as hallucination and delusion, they, regardless of the generations of antipsychotics, have only limited efficacy to the cognitive dysfunction and the negative symptoms such as apathy and social withdrawal. To aim to treat the cognitive dysfunction and negative symptoms, various targets such as glutamate receptors, tachykinin receptors, cannabinoid receptors, and nicotinic acetylcholine receptors, have been investigated recently. PET technique with the radioligands developed for these targets has potentials to rationalize and speed up the process of the drug development for the treatment of schizophrenia.