In computing, overclocking is the practice of increasing the clock rate of a semiconductor device, such as a processor, beyond its rated speed, potentially increasing its performance. Overclocked devices, however, may have shorter lifespans, become unstable and unreliable, and in extreme cases, be permanently damaged. Many manufacturers do not cover damage from overclocking in their warranties, while some allow it inside a predefined safety margin.

A semiconductor device's processing speed depends on a variety of factors, including, but not limited to, its clock speed, microarchitecture, the kind of software it's running, and the bandwidth, latency and size for each level of its memory. All else being equal, a faster-clocked device can, though not necessarily, perform faster. Operating voltage is often increased to maintain a component's operational stability at accelerated speeds. Operating at higher frequencies and voltages increase power consumption and heat. Overclocking a device introduces additional risks of failure, for example, by overheating when the increased heat load is not removed, or by the device requesting more power than its power supply can provide.

Conversely, the primary goal of underclocking is to reduce power consumption and the resultant heat generation of a device, with the trade-offs being lower clock speeds and reductions in performance. Reducing the cooling requirements needed to keep hardware at a given operational temperature has knock-on benefits such as lowering the number and speed of fans to allow quieter operation, and in mobile devices increase the length of battery life per charge. Some manufacturers underclock components of battery-powered equipment to improve battery life, or implement systems that detect when a device is operating under battery power and reduce clock frequency.

Underclocking and undervolting would be attempted on a desktop system to have it operate silently (such as for a home entertainment center) while potentially offering higher performance than currently offered by low-voltage processor offerings. This would use a "standard-voltage" part and attempt to run with lower voltages (while attempting to keep the desktop speeds) to meet an acceptable performance/noise target for the build. This was also attractive as using a "standard voltage" processor in a "low voltage" application avoided paying the traditional price premium for an officially certified low voltage version. However again like overclocking there is no guarantee of success, and the builder's time researching given system/processor combinations and especially the time and tedium of performing many iterations of stability testing need to be considered. The usefulness of underclocking (again like overclocking) is determined by what processor offerings, prices, and availability are at the specific time of the build. Underclocking is also sometimes used when troubleshooting.

Overclocking has become more accessible with motherboard makers offering overclocking as a marketing feature on their mainstream product lines. However, the practice is embraced more by enthusiasts than professional users, as overclocking carries a risk of reduced reliability, accuracy and damage to data and equipment. Additionally, most manufacturer warranties and service agreements do not cover overclocked components nor any incidental damages caused by their use. While overclocking can still be an option for increasing personal computing capacity, and thus workflow productivity for professional users, the importance of stability testing components thoroughly before employing them into a production environment cannot be overstated.

Overclocking offers several draws for overclocking enthusiasts. Overclocking allows testing of components at speeds not currently offered by the manufacturer, or at speeds only officially offered on specialized, higher-priced versions of the product. A general trend in the computing industry is that new technologies tend to debut in the high-end market first, then later trickle down to the performance and mainstream market. If the high-end part only differs by an increased clock speed, an enthusiast can attempt to overclock a mainstream part to simulate the high-end offering. This can give insight on how over-the-horizon technologies will perform before they are officially available on the mainstream market, which can be especially helpful for other users considering if they should plan ahead to purchase or upgrade to the new feature when it is officially released.

Some hobbyists enjoy building, tuning, and "Hot-Rodding" their systems in competitive benchmarking competitions, competing with other like-minded users for high scores in standardized computer benchmark suites. Others will purchase a low-cost model of a component in a given product line, and attempt to overclock that part to match a more expensive model's stock performance. Another approach is overclocking older components to attempt to keep pace with increasing system requirements and extend the useful service life of the older part or at least delay purchase of new hardware solely for performance reasons. Another rationale for overclocking older equipment is even if overclocking stresses equipment to the point of failure earlier, little is lost as it is already depreciated, and would have needed to be replaced in any case.

Stock cooling systems are designed for the amount of power produced during non-overclocked use; overclocked circuits can require more cooling, such as by powerful fans, larger heat sinks, heat pipes and water cooling. When maximum cooling is not required, fan speeds can be reduced to below the maximum. Fan noise has been found to be roughly proportional to the fifth power of fan speed, and halving speed reduces noise by about 15 dB. Mass, shape, and material all influence the ability of a heatsink to dissipate heat. Efficient heatsinks are often made entirely of copper, which has high thermal conductivity, but is expensive. Aluminium is more widely used; it has good thermal characteristics, though not as good as copper, and is significantly cheaper. Cheaper materials such as steel do not have good thermal characteristics. Heat pipes can be used to improve conductivity. Many heatsinks combine two or more materials to achieve a balance between performance and cost.