The only bromine and iodine radioisotopes worth using in PET or SPECT in vivo investigations during the development of a new drug are 76Br and 123I. It is most of the time impossible to isotopically label a drug with 76Br or 123I since the occurrence of drugs having a bromine or an iodine atom within their chemical structure is quite limited. However, by using specific radiobrominated or radioiodinated probes, it is possible to study in vivo the potential interaction of a drug with biochemical processes such as blood flow, glucose consumption, protein synthesis or cell proliferation and neurotransmission. Radiobrominated and radioiodinated probes have been described mainly for assessing cell proliferation. For imaging various classes of specific binding sites involved in neuronal or hormonal transmission, a great number of radiohalogenated ligands have been proposed and validated. The two-steps strategy consists of performing an "in vivo assay" by using first of all, one of these specific radio-brominated/-iodinated ligands (or probes) for targeting specific binding sites (receptor, transporter, enzymes) and in a second step by assessing the interaction of the cold drug on the binding of these probes. This indirect observation of drug-receptor (transporter, enzyme) occupancy allows predicting response, optimum dose and optimum scheduling. The most important radiobrominated and radioiodinated ligands specific for dopaminergic, serotoninergic, cholinergic and gabaergic binding sites and their application in drug development processes are reviewed.