Handloading, or reloading, is the practice of making firearm cartridges by manually assembling the individual components (metallic/polymer case, primer, propellant and projectile), rather than purchasing mass-assembled, factory-loaded commercial ammunition. (It should not be confused with the reloading of a firearm with cartridges, such as by swapping detachable magazines, or using a stripper clip or speedloader to quickly insert new cartridges into a magazine.)

The term handloading is the more general term, and refers generically to the manual assembly of ammunition cartridges. Reloading refers more specifically to handloading using previously fired cases and shotshells. The terms are often used interchangeably however, as the techniques are largely the same, whether the handloader is using new or recycled components. The differences lie in the initial preparation of cases or shells — new components are generally ready to load straight out of the box, while previously fired components often need additional preparation procedures, such as removal of expended primers ("depriming"), case cleaning (to remove any fouling or rust) and the reshaping (to correct any pre-existing deformations) and resizing of cases to bring them back into specification after firing (or to experiment with custom modifications).

Some consider handloading cartridges or shotshells a hobby, but reloading can save the shooter money, providing a greater quantity of higher quality ammunition within a given budget. However, hobbyists' enjoyment of the reloading process may be a significant benefit.

Handloading ammunition avoids the labor costs of commercial production lines, reducing the expenditure to only the cost of purchasing components and equipment. Reloading may not be cost effective for occasional shooters, as it takes time to recoup the cost of needed equipment, but those who shoot more frequently will see cost-savings over time, as the brass cartridge cases and shotgun shell hulls, which are often the most expensive components, can be reused with proper maintenance.

There are three aspects to ballistics: internal ballistics, external ballistics, and terminal ballistics. Internal ballistics refers to things that happen inside the firearm during and after firing, but before the bullet leaves the muzzle. The handloading process can realize increased accuracy and precision through improved consistency of manufacture, by selecting the optimal bullet weight and design, and tailoring bullet velocity to the purpose. Each cartridge reloaded can have each component carefully matched to the rest of the cartridges in the batch. Brass cases can be matched by volume, weight, and concentricity, bullets by weight and design, powder charges by weight, type, case filling (amount of total usable case capacity filled by charge), and packing scheme (characteristics of granule packing).

Handloading is a fundamental prerequisite for success where the most extreme accuracy is demanded, such as in rifle benchrest shooting, but can only be done consistently accurately following load development to determine what cartridge components work best with a specific rifle.

Customized performance is a common goal of handloaders. Hunters may desire cartridges with specialized bullets with specific terminal performance. Target shooters often experiment extensively with component combinations in an effort to achieve the best and most consistent bullet trajectories, often using cartridge cases that have been fire formed in order to best fit the chamber of a specific firearm. Shotgun shooters can make specialty rounds unavailable through commercial inventories at any price. Some handloaders even customize cartridges and shotshells to reduce recoil for shooters who might otherwise avoid shooting sports because of the high recoil of certain firearms. Some handloaders make increased-power ammunition (i.e. "hot loads") if higher muzzle velocities (hence flatter trajectories) are desired.

The equipment used to assemble the cartridge has an effect on its uniformity/consistency and optimal shape/size. Dies used to size the cartridges can be matched to the chamber of a given gun. Modern handloading equipment enables a firearm owner to tailor fresh ammunition to a specific firearm, and to precisely measured tolerances far improving the comparatively wide tolerances within which commercial ammunition manufacturers must operate.