This chapter provides an overview of the GE Dual-Energy Approach to Spectral Imaging known as Gemstone Spectral Imaging (GSI). Since 2010 when GE launched GSI on Discovery™ CT750 HD, GE has provided continued improvements to the technology, capabilities, and workflow, as well as spectral clinical tools to support advances in diagnosis and solutions across the Discovery™ GSI, Revolution™ CT Frontier, and Revolution™ CT/Revolution™ Apex platforms. The GSI acquisition method enables precise temporal registration of the dual-energy sinograms, projection-based material decomposition, and delivers a full 50 cm material decomposition scan field of view. The technologies employed to achieve the dual-energy acquisitions are detailed in the discussion of balanced system design below. Calibration of fast kVp switching data, material decomposition, and visualization of the resulting images are covered in the image reconstruction, projection-based material decomposition, and post-processing/clinical applications sections. The chapter closes with GSI implementation in the context of workflow, imaging improvements, solutions to challenging diagnostic applications, and the path to the future of photon counting. Academic research and clinical exploration of GSI have grown rapidly and resulted in rich scientific publications and extensive clinical adoption. GSI has shown benefits in: Better lesion characterization by providing information about the chemical composition and material characteristicsImproving lesion detection with enhanced contrast-to-noise ratioReducing beam hardening and metal artifactsOptimizing iodine load in contrast enhanced CT studies.