This chapter provides an in-depth overview of image formation principles in nuclear medicine, focusing on essential factors for achieving high-quality images that support accurate diagnosis and therapeutic monitoring. It covers the technical and biological components that influence both image quality and quantitative accuracy, including critical parameters such as matrix size, acquisition timing, reconstruction algorithms, and calibration. The chapter emphasizes the role of quality control (QC) protocols, such as daily calibration and uniformity checks, in maintaining image consistency and reliability across single photon emission computed tomography (SPECT) and positron emission tomography (PET) systems. Differences between diagnostic images used for visual assessment and those processed for quantification are examined, highlighting how these variations impact clinical interpretations, particularly in measurements like standardized uptake value (SUV). By understanding and applying these foundational principles, radiologists and nuclear medicine professionals can ensure that PET/CT and SPECT/CT imaging provide precise, clinically valuable data for improved diagnostic accuracy and treatment planning.