The EANM guidance document provides recommendations for dosimetry in first-in-human studies and early phase clinical trials of novel nuclear medicine agents. It includes an introduction to different emitters and carrier molecules, followed by recommendations on activity measurement, pharmacokinetic analysis, absorbed dose calculations, and uncertainty analysis. The document discusses the optimal use of preclinical information and studies involving diagnostic analogues, emphasizes good practice reporting, and lists relevant dosimetry parameters and method descriptions. Three examples of first-in-human dosimetry studies are given, including both diagnostic tracers and radionuclide therapies. The document highlights the importance of dosimetry in drug development, as regulatory bodies such as the EMA and FDA require it for marketing authorization. It also addresses the challenges of dosimetry for novel emitters and carrier molecules, the need for careful selection of measurement procedures, and the importance of considering animal studies for extrapolation to humans. The document outlines the principles of dosimetry, including the calculation of absorbed dose, equivalent dose, and effective dose, and provides guidance on the use of different models and methods for dosimetry calculations. It also discusses the importance of uncertainty estimation in dosimetry and the need for accurate and reliable data in first-in-human studies. The document emphasizes the importance of proper measurement protocols, including the selection of source organs, the use of appropriate imaging techniques, and the consideration of various factors such as the administration route and the physical and biological aspects of the radiopharmaceutical. The document also provides guidance on the use of anatomical imaging for accurate estimation of tissue masses and the importance of considering the biodistribution and time-integrated activity of the radiopharmaceutical. Overall, the document aims to provide a comprehensive guide for the proper conduct of dosimetry studies in first-in-human and early phase clinical trials of novel nuclear medicine agents.The EANM guidance document provides recommendations for dosimetry in first-in-human studies and early phase clinical trials of novel nuclear medicine agents. It includes an introduction to different emitters and carrier molecules, followed by recommendations on activity measurement, pharmacokinetic analysis, absorbed dose calculations, and uncertainty analysis. The document discusses the optimal use of preclinical information and studies involving diagnostic analogues, emphasizes good practice reporting, and lists relevant dosimetry parameters and method descriptions. Three examples of first-in-human dosimetry studies are given, including both diagnostic tracers and radionuclide therapies. The document highlights the importance of dosimetry in drug development, as regulatory bodies such as the EMA and FDA require it for marketing authorization. It also addresses the challenges of dosimetry for novel emitters and carrier molecules, the need for careful selection of measurement procedures, and the importance of considering animal studies for extrapolation to humans. The document outlines the principles of dosimetry, including the calculation of absorbed dose, equivalent dose, and effective dose, and provides guidance on the use of different models and methods for dosimetry calculations. It also discusses the importance of uncertainty estimation in dosimetry and the need for accurate and reliable data in first-in-human studies. The document emphasizes the importance of proper measurement protocols, including the selection of source organs, the use of appropriate imaging techniques, and the consideration of various factors such as the administration route and the physical and biological aspects of the radiopharmaceutical. The document also provides guidance on the use of anatomical imaging for accurate estimation of tissue masses and the importance of considering the biodistribution and time-integrated activity of the radiopharmaceutical. Overall, the document aims to provide a comprehensive guide for the proper conduct of dosimetry studies in first-in-human and early phase clinical trials of novel nuclear medicine agents.