This article is an introductory survey of non-equilibrium critical phenomena at phase transitions into absorbing states, i.e., states which a system can never escape from once it entered. Scaling laws governing such transitions are reviewed, in particular for the case of the fundamental universality class called the directed percolation class, on the basis of an experimental example found in liquid-crystal turbulence. Recent experimental developments on absorbing-state transitions are also discussed, which concern fluid systems, colloid suspensions, and superconductor vortices.