Molecular imaging is a rapidly emerging field that extends morphological observations to a more meaningful dimension by visualizing, characterizing, and quantifying biological processes at the cellular and subcellular levels within intact living organisms. This multidisciplinary approach integrates techniques from molecular biology, chemistry, medicine, pharmacology, medical physics, biomathematics, and bioinformatics. Unlike traditional imaging methods that rely on nonspecific physical, physiological, or metabolic changes, molecular imaging uses specific molecular probes to identify specific molecular events, such as gene expression, protein interactions, and drug effects. This shift from nonspecific to specific imaging has significant implications for understanding integrative biology, early disease detection, and treatment evaluation. The development of molecular imaging strategies is driven by advances in molecular and cell biology, transgenic animal models, and the availability of highly specific imaging drugs and probes. These advancements have enabled the creation of imaging methods that reflect specific cellular and molecular processes, monitor multiple molecular events simultaneously, and follow cell trafficking and targeting. Molecular imaging also offers advantages over conventional in vitro and cell culture techniques, such as the ability to assess phenotypic changes in transgenic and gene-targeted animal models without killing the animals, and the potential for rapid, reproducible, and quantitative data collection. Molecular imaging techniques include radionuclide imaging (e.g., PET and SPECT), optical imaging (e.g., bioluminescence and fluorescence), magnetic resonance imaging (MRI), computed tomography (CT), and ultrasonography. Each technique has its unique advantages and limitations, such as spatial and temporal resolution, depth penetration, energy expenditure, and the availability of injectable/biocompatible molecular probes. For example, PET provides high sensitivity and quantitative information but is less sensitive than SPECT. Optical imaging is quick and easy but has limited depth penetration and lacks an equivalent human imaging modality. MRI offers high spatial resolution and simultaneous extraction of physiological/molecular and anatomical information but is less sensitive than radionuclide and optical techniques. The future of molecular imaging lies in the development of more advanced imaging technologies, the integration of multiple imaging modalities, and the creation of novel contrast agents to enhance the detection of specific molecular events. These advancements will enable more precise and comprehensive understanding of biological processes and improve the diagnosis and treatment of diseases.Molecular imaging is a rapidly emerging field that extends morphological observations to a more meaningful dimension by visualizing, characterizing, and quantifying biological processes at the cellular and subcellular levels within intact living organisms. This multidisciplinary approach integrates techniques from molecular biology, chemistry, medicine, pharmacology, medical physics, biomathematics, and bioinformatics. Unlike traditional imaging methods that rely on nonspecific physical, physiological, or metabolic changes, molecular imaging uses specific molecular probes to identify specific molecular events, such as gene expression, protein interactions, and drug effects. This shift from nonspecific to specific imaging has significant implications for understanding integrative biology, early disease detection, and treatment evaluation. The development of molecular imaging strategies is driven by advances in molecular and cell biology, transgenic animal models, and the availability of highly specific imaging drugs and probes. These advancements have enabled the creation of imaging methods that reflect specific cellular and molecular processes, monitor multiple molecular events simultaneously, and follow cell trafficking and targeting. Molecular imaging also offers advantages over conventional in vitro and cell culture techniques, such as the ability to assess phenotypic changes in transgenic and gene-targeted animal models without killing the animals, and the potential for rapid, reproducible, and quantitative data collection. Molecular imaging techniques include radionuclide imaging (e.g., PET and SPECT), optical imaging (e.g., bioluminescence and fluorescence), magnetic resonance imaging (MRI), computed tomography (CT), and ultrasonography. Each technique has its unique advantages and limitations, such as spatial and temporal resolution, depth penetration, energy expenditure, and the availability of injectable/biocompatible molecular probes. For example, PET provides high sensitivity and quantitative information but is less sensitive than SPECT. Optical imaging is quick and easy but has limited depth penetration and lacks an equivalent human imaging modality. MRI offers high spatial resolution and simultaneous extraction of physiological/molecular and anatomical information but is less sensitive than radionuclide and optical techniques. The future of molecular imaging lies in the development of more advanced imaging technologies, the integration of multiple imaging modalities, and the creation of novel contrast agents to enhance the detection of specific molecular events. These advancements will enable more precise and comprehensive understanding of biological processes and improve the diagnosis and treatment of diseases.