Positron emission tomography (PET) visualization of brain components in vivo is a rapidly growing field. Molecular imaging with PET is also increasingly used in drug development, especially for the determination of drug receptor interaction for CNS-active drugs. This gives the opportunity to relate clinical efficacy to per cent receptor occupancy of a drug on a certain targeted receptor and to relate drug pharmacokinetics in plasma to interaction with target protein. In the present review we will focus on the study of transporters, such as the monoamine transporters, the P-glycoprotein (Pgp) transporter, the vesicular monoamine transporter type 2, and the glucose transporter using PET radioligands. Neurotransmitter transporters are presynaptically located and in vivo imaging using PET can therefore be used for the determination of the density of afferent neurons. Several promising PET ligands for the noradrenaline transporter (NET) have been labeled and evaluated in vivo including in man, but a really useful PET ligand for NET still remains to be identified. The most promising tracer to date is (S,S)-[¹⁸F]FMeNER-D₂. The in vivo visualization of the dopamine transporter (DAT) may give clues in the evaluation of conditions related to dopamine, such as Parkinson's disease and drug abuse. The first PET radioligands based on cocaine were not selective, but more recently several selective tracers such as [¹¹C]PE2I have been characterized and shown to be suitable as PET radioligands. Although there are a large number of serotonin transporter inhibitors used today as SSRIs, it was not until very recently, when [¹¹C]McN5652 was synthesized, that this transporter was studied using PET. New candidates as PET radioligands for the SERT have subsequently been developed and [¹¹C]DASB and [¹¹C]MADAM and their analogues are today the most promising ligands. The existing radioligands for Pgp transporters seem to be suitable tools for the study of both peripheral and central drug–Pgp interactions, although [¹¹C]verapamil and [¹⁸F]fluoropaclitaxel are probably restricted to use in studies of the blood–brain barrier. The vesicular monoamine transporter 2 (VMAT2) is another interesting target for diagnostic imaging and [¹¹C]DTBZ is a promising tracer. The noninvasive imaging of transporter density as a function of disease progression or availability following interaction with blocking drugs is highlighted, including the impact on both development of new therapies and the process of developing new drugs. Although CNS-related work focusing on psychiatric disorders is the main focus of this review, other applications of PET ligands, such as diagnosis of cancer, diabetes research, and drug interactions with efflux systems, are also discussed. The use of PET especially in terms of tracer development is briefly described. Finally, it can be concluded that there is an urgent need for new, selective radioligands for the study of the transporter systems in the human brain using PET.