In this chapter, we summarise the methods for the measurement of platelet secretion from their internal stores: α-, δ-granules, and lysosomes. We present the main disorders linked to each granule and discuss the advantages and limitations of diagnostic assays currently used within the clinic, as well as recent advancements within laboratory research. We describe the basis and use of reporter and immune-based assays including enzyme-linked immunosorbent assay (ELISA), flow cytometry, and Western blotting, as well as microscopy imaging. We also discuss recent developments in research technologies which have contributed to the in-depth study of platelet granules composition, mechanisms of secretion, and their interactions within the vascular and immune systems. Many of these techniques however are still confined to research laboratories and have not yet transitioned to clinical practice owing to challenges such as standardisation issues, high costs, and technical complexity requiring specialised expertise and equipment. Finally, we present case studies illustrating the use of both current clinical and research methods towards understanding and diagnosing platelet secretion disorders.