Aid climbing is a style of climbing in which fixed or placed protection is used to make upward progress. In the Yosemite Decimal System used in the US, it is sometimes called "6th class" climbing.
The term contrasts with free climbing in which no artificial aids are used to make progress, just the climber's hands and feet. Aid climbing places less emphasis on athletic fitness and physical strength but more on technical skill.
In a typical ascent with aid the climber places protection, then clips an aider (a ladder-like device, also called stirrup or étrier) to the protection, stands up on the aider and repeats the process. Until the 1940s the only protection was the piton, driven into a crack in the rock with a hammer.
Today, aid climbing uses a considerably larger array of hardware than the pitons used by the first climbers although the primary technique of ascension has not much evolved. The typical gear of an aid climber includes pitons, hooks, copperheads, nuts, camming devices, ascenders, hauling pulleys, aiders, daisy chains and wall hammers. The invention of camming devices and other non-damaging rock gear has resulted in the practice of clean aid, where nothing is hammered, a great bonus for popular routes which could be disfigured from continual hammering.
Until the 1960s or so, aid climbing was normal practice in most climbing areas. But as improvements in technique and equipment meant that many aid routes could be climbed free, some influential climbers began to criticise the use of aid as being against the spirit of mountaineering. Reinhold Messner wrote, "Rock faces are no longer overcome by climbing skill, but are humbled, pitch by pitch, by methodical manual labour … Who has polluted the pure spring of mountaineering?" (from "The Murder of the Impossible").
Free climbing is now the mainstream of climbing. But aid climbers have answered the criticism of Messner and others by climbing routes where the absence of holds or features in the rock make free climbing impossible, and by eschewing purely mechanical techniques (such as repetitively drilling bolts). The hardest aid routes are poorly protected and the climber relies exclusively on the gear that he/she has placed.