Yield stress is the stress level where a material’s elastic behaviour gives way to viscous behaviour — below it the material holds its shape like a solid, above it the material flows. It marks the point where yield strength is exceeded; keep loading and the material may reach failure stress, beyond which it breaks. Rheologists measure it in Newtons or Pascals. Food scientists meet it in pastes, gels, creams, ketchup, and chocolate: ketchup’s refusal to leave the bottle until shaken or squeezed is its yield stress at work. Formulators tune it for spreadability, stability against separation, and mouthfeel, with rheometers measuring it as a routine specification for structured products.