Platelets play a key role in many physiological and pathological processes. In response to stimuli, platelets become activated with dynamic changes to their ultrastructure occurring. These include changes in morphology, granule release, cytoskeletal reorganisation, organelle remodelling, receptor and lipid redistribution, and extracellular vesicle formation. Knowledge of these alterations in platelet morphology and ultrastructure allows an understanding of how platelets can adapt to, and perform, a large range of diverse functions. This understanding can also help discern the aetiology of platelet disorders, develop treatments and identify biomarkers of disease. This chapter will give an overview of the resting platelet ultrastructure, how this changes during platelet activation and how these changes can be visualised with increasing spatial and temporal resolution.