A standard candle is an astronomical object whose intrinsic brightness is known, so that how faint it appears reveals how far away it is. Cepheid variable stars serve at moderate distances, pulsating with a period tied directly to their luminosity, while Type Ia supernovae reach across billions of light-years. Chaining these methods together builds the cosmic distance ladder, each rung calibrated against the one below. Persistent disagreement between ladder-based measurements and those from the early universe is the heart of the Hubble tension.