[en] Experience in Member States has shown that frequent radiation monitoring at borders or within a country results in a significant number of radiation alarms, where most of them are of an innocent nature, i.e. caused by naturally occurring radioactive material, by pharmaceutical radionuclides in persons who have had medical treatment or by legal radioactive shipments. The rapid resolution of innocent alarms, with an acceptably low level of vulnerability to overlooking a threat isotope, is one of the most important tasks in radiation monitoring. In this process, the impact on the free flow of goods and passengers must be minimized. This problem area is also the driving force for R and D in the field of radiological security. Since frontline officers (FLOs), i.e. customs, police, border guards and security, are not experts in radiation detection, practice in Member States has shown that they can be successful only if reliable expert support is available. If not, the FLOs may become insecure and may not be able to carry out radiation monitoring effectively. How this support is set up by different Member States differs. There are several areas where expert support is needed: initial detection phase, emergency response if required, nuclear forensics and attribution, not to forget threat assessment, equipment purchasing, installation, maintenance and training. In the paper, the mobile expert support team (MEST) concept is discussed, as introduced in Member States, supported by the IAEA to provide support in the initial detection phase and often also in other areas discussed. It consists of various components, such as equipment and techniques used by analytical experts and radiological assessors, procedures and a training programme, emphasizing the operative interaction between FLOs and the MEST. (author)