This document provides guidance for dosimetry in first-in-human and early-phase clinical trials of novel nuclear medicine agents. It covers the selection of measurement procedures, methods for activity measurement, pharmacokinetic analyses, absorbed dose calculations, and uncertainty analyses. The document emphasizes the importance of preclinical information and studies involving diagnostic analogues, and outlines good practice reporting standards. It includes examples of first-in-human dosimetry studies for both diagnostic tracers and radionuclide therapies. Key topics include the selection of radionuclides and carrier molecules, administration routes, and the use of imaging techniques for biodistribution studies. The document also discusses the challenges and considerations in extrapolating animal data to humans, the importance of flexible measurement protocols, and the need for standardized reporting in multicenter studies. Additionally, it provides detailed guidance on blood sampling, urine and fecal sample collection, and whole-body probe measurements. The document concludes with recommendations for absorbed dose calculations, including definitions of absorbed dose, equivalent dose, and effective dose, and specific considerations for therapeutic and diagnostic applications.This document provides guidance for dosimetry in first-in-human and early-phase clinical trials of novel nuclear medicine agents. It covers the selection of measurement procedures, methods for activity measurement, pharmacokinetic analyses, absorbed dose calculations, and uncertainty analyses. The document emphasizes the importance of preclinical information and studies involving diagnostic analogues, and outlines good practice reporting standards. It includes examples of first-in-human dosimetry studies for both diagnostic tracers and radionuclide therapies. Key topics include the selection of radionuclides and carrier molecules, administration routes, and the use of imaging techniques for biodistribution studies. The document also discusses the challenges and considerations in extrapolating animal data to humans, the importance of flexible measurement protocols, and the need for standardized reporting in multicenter studies. Additionally, it provides detailed guidance on blood sampling, urine and fecal sample collection, and whole-body probe measurements. The document concludes with recommendations for absorbed dose calculations, including definitions of absorbed dose, equivalent dose, and effective dose, and specific considerations for therapeutic and diagnostic applications.