Nanoparticle technologies are significantly impacting the development of both therapeutic and diagnostic agents. Theranostics, combining therapeutic and diagnostic functions, is a promising approach for personalized medicine, particularly for targeting molecular biomarkers. This review discusses state-of-the-art nanoparticles for therapeutic and diagnostic purposes, highlighting challenges in integrating these fields. Major nanoparticle classes include drug conjugates, dendrimers, vesicles, micelles, core-shell particles, microbubbles, and carbon nanotubes. These nanoparticles can carry drugs, contrast agents, or both, enabling simultaneous imaging and treatment. Various imaging modalities, such as optical, MRI, SPECT, PET, CT, and ultrasound, are used to monitor nanoparticle interactions with disease. Each modality has unique advantages and limitations. Nanoparticles offer opportunities for passive and active targeting, environmental-responsive drug release, and molecular imaging. However, challenges include biomarker discovery, nanoparticle toxicity, formulation stability, production costs, and intellectual property. The development of theranostic nanoparticles requires balancing imaging sensitivity, targeting accuracy, and controlled drug release. Current research explores various materials and pathways to achieve these goals. Nanoparticles are well-suited for targeted contrast agents due to their surface functionalization, tunable circulation time, and ability to incorporate drugs and contrast agents. Different nanoparticle types, such as drug conjugates, dendrimers, vesicles, micelles, core-shell particles, microbubbles, and carbon nanotubes, are being developed for theranostic applications. Challenges include toxicity, biodistribution, and biodegradation. Despite these challenges, theranostic nanoparticles offer potential for improved diagnosis and treatment of diseases, particularly cancer. However, safety and complexity issues must be addressed before clinical translation.Nanoparticle technologies are significantly impacting the development of both therapeutic and diagnostic agents. Theranostics, combining therapeutic and diagnostic functions, is a promising approach for personalized medicine, particularly for targeting molecular biomarkers. This review discusses state-of-the-art nanoparticles for therapeutic and diagnostic purposes, highlighting challenges in integrating these fields. Major nanoparticle classes include drug conjugates, dendrimers, vesicles, micelles, core-shell particles, microbubbles, and carbon nanotubes. These nanoparticles can carry drugs, contrast agents, or both, enabling simultaneous imaging and treatment. Various imaging modalities, such as optical, MRI, SPECT, PET, CT, and ultrasound, are used to monitor nanoparticle interactions with disease. Each modality has unique advantages and limitations. Nanoparticles offer opportunities for passive and active targeting, environmental-responsive drug release, and molecular imaging. However, challenges include biomarker discovery, nanoparticle toxicity, formulation stability, production costs, and intellectual property. The development of theranostic nanoparticles requires balancing imaging sensitivity, targeting accuracy, and controlled drug release. Current research explores various materials and pathways to achieve these goals. Nanoparticles are well-suited for targeted contrast agents due to their surface functionalization, tunable circulation time, and ability to incorporate drugs and contrast agents. Different nanoparticle types, such as drug conjugates, dendrimers, vesicles, micelles, core-shell particles, microbubbles, and carbon nanotubes, are being developed for theranostic applications. Challenges include toxicity, biodistribution, and biodegradation. Despite these challenges, theranostic nanoparticles offer potential for improved diagnosis and treatment of diseases, particularly cancer. However, safety and complexity issues must be addressed before clinical translation.