This review discusses the development and applications of gold nanoshells (AuNSs) as theranostic agents for cancer diagnosis and treatment. AuNSs, which are silica-coated gold nanoparticles, have unique optical properties that allow them to function as both diagnostic and therapeutic tools. They can be tuned to absorb or scatter light in the near-infrared (NIR) range, enabling photothermal therapy and enhancing fluorescence imaging. The review highlights the design and synthesis of AuNS-based theranostic agents, their optical properties, and their applications in imaging, photothermal ablation, and gene therapy. The photothermal properties of AuNSs are due to their ability to absorb light resonant with their plasmon energy, converting it into heat. This heat can be used to destroy cancer cells. The review also discusses the enhancement of fluorescence from weak NIR fluorophores by AuNSs, which can improve the sensitivity of fluorescence imaging. AuNSs have been used as contrast agents in optical coherence tomography (OCT) and photoacoustic imaging, and as multimodal theranostic probes combining near-infrared fluorescence, magnetic resonance imaging (MRI), and photothermal therapy. The review also explores the use of AuNSs as gene therapy vectors. Their plasmonic properties allow for light-controlled release of DNA, enabling precise temporal control over gene delivery. The review describes the fabrication of DNA-conjugated AuNSs and compares the efficiency of light-induced and thermally-induced DNA release. It also examines the light-triggered release of DAPI molecules, which bind to DNA, to visualize intracellular light-induced release. The review discusses the potential of AuNSs in in vivo applications, including their biodistribution and therapeutic efficacy in animal models. It also highlights the integration of multiple functions in a single nanocomplex, such as targeting, imaging, and therapy, which can provide valuable biodistribution information and improve therapeutic outcomes. The review concludes with a discussion of the future prospects of nanoshell-based theranostics, emphasizing their potential impact and challenges in the next decade.This review discusses the development and applications of gold nanoshells (AuNSs) as theranostic agents for cancer diagnosis and treatment. AuNSs, which are silica-coated gold nanoparticles, have unique optical properties that allow them to function as both diagnostic and therapeutic tools. They can be tuned to absorb or scatter light in the near-infrared (NIR) range, enabling photothermal therapy and enhancing fluorescence imaging. The review highlights the design and synthesis of AuNS-based theranostic agents, their optical properties, and their applications in imaging, photothermal ablation, and gene therapy. The photothermal properties of AuNSs are due to their ability to absorb light resonant with their plasmon energy, converting it into heat. This heat can be used to destroy cancer cells. The review also discusses the enhancement of fluorescence from weak NIR fluorophores by AuNSs, which can improve the sensitivity of fluorescence imaging. AuNSs have been used as contrast agents in optical coherence tomography (OCT) and photoacoustic imaging, and as multimodal theranostic probes combining near-infrared fluorescence, magnetic resonance imaging (MRI), and photothermal therapy. The review also explores the use of AuNSs as gene therapy vectors. Their plasmonic properties allow for light-controlled release of DNA, enabling precise temporal control over gene delivery. The review describes the fabrication of DNA-conjugated AuNSs and compares the efficiency of light-induced and thermally-induced DNA release. It also examines the light-triggered release of DAPI molecules, which bind to DNA, to visualize intracellular light-induced release. The review discusses the potential of AuNSs in in vivo applications, including their biodistribution and therapeutic efficacy in animal models. It also highlights the integration of multiple functions in a single nanocomplex, such as targeting, imaging, and therapy, which can provide valuable biodistribution information and improve therapeutic outcomes. The review concludes with a discussion of the future prospects of nanoshell-based theranostics, emphasizing their potential impact and challenges in the next decade.