This document provides an extensive overview of the state of the art in research, development and use of low-flying UAVs in radiological and nuclear emergencies. The main driver behind this study is the G516 gap listed in the dynamic catalogue of the ENCIRCLE project but it is also referenced to the IFAFRI capability gaps. After an overview of possible intervention scenarios where low flying UAVs can improve the detection, identification and monitoring capabilities of First Responders, the document critically revises the current achievements in integrating radiation system measurements on UAVs. In particular the study considers some explicative examples of results from R&D projects/activities and more advanced solutions already mature for a commercial exploitation. Critical for the entire study is the reference to the EU Regulations 2019/947 and 2019/945 that set out the framework for the safe operation of civil drones in the European skies. The document also overviews actual barrier for further penetration of the technology in the security market and standardization issues. Finally, it describes lessons learnt during INCLUDING project exercise and training activities where UAVs with radiation measurements device onboard were deployed in simulations close to real operational scenarios. With new solutions and advances progressing at rapid pace, the document is intended as a snapshot taken at the moment of its release but that will be updated in due course during INCLUDING project execution.