Prototyping turns speculation into evidence. We map modalities—physical, digital, service, and experience prototypes—and show how to choose the right fidelity to answer the next question fast. Craftsmanship is foregrounded: sketching, building, instrumenting, and refining in contact with materials, users, and constraints. With examples from additive-manufacturing-enabled iteration and data-rich testing, the chapter frames prototypes as boundary objects that align stakeholders, de-risk decisions, and accelerate learning.