Toner is a powder mixture used in laser printers and photocopiers to form the text and images on paper, in general through a toner cartridge. Mostly granulated plastic, early mixtures added only carbon powder and iron oxide; now there are mixtures that contain polypropylene, fumed silica, and various minerals for triboelectrification. Toner using plant-derived plastic also exists as an alternative to petroleum plastic. Toner particles are melted by the heat of the fuser, and are thus bonded to the paper.

In earlier photocopiers, this low-cost carbon toner was poured by the user from a bottle into a reservoir in the machine.^{[citation needed]} Later copiers, and laser printers from the first 1984 Hewlett-Packard LaserJet, feed directly from a sealed toner cartridge.

Laser toner cartridges for use in color copiers and printers come in sets of cyan, magenta, yellow and black (CMYK), allowing a very large color gamut to be generated by mixing.

The specific polymer used varies by manufacturer but can be a styrene acrylate copolymer, a polyester resin, a styrene butadiene copolymer, or a few other special polymers. Toner formulations vary from manufacturer to manufacturer and even from machine to machine. Typically formulation, granule size and melting point vary the most.

Originally, the particle size of toner averaged 14–16 micrometres (μm) or greater. Theoretically, for the perfect reproduction of dots and print features at 600 dpi, a particle size of about 5 μm is required and, at 1200 dpi, about 3 μm is required. Further reductions in particle size producing further improvements in resolution are being developed through the application of new technologies such as Emulsion-Aggregation. Toner manufacturers maintain a quality control standard for particle size distribution in order to produce a powder suitable for use in their printers.

Toner has traditionally been made by compounding the ingredients and creating a slab which was broken or pelletized, then turned into a fine powder with a controlled particle size range by air jet milling. This process results in toner granules with varying sizes and aspherical shapes. To get a finer print, some companies are using a chemical process to grow toner particles from molecular reagents. This results in more uniform size and shapes of toner particles. The smaller, uniform shapes permit more accurate colour reproduction and more efficient toner use.

Toner can be washed off skin and garments with cold water. Hot or warm water softens the toner, causing it to bond in place. Toner fused to skin eventually wears off, or can be partially removed using an abrasive hand cleaner. Toner fused to clothing usually cannot be removed. Unfused toner is easily cleaned from most water-washable clothing. Because toner is a wax or plastic powder with a low melting temperature it must be kept cold while cleaning.

Toner particles have electrostatic properties by design and can develop static-electric charges when they rub against other particles, objects, or the interiors of transport systems and vacuum cleaner hoses. Because of this and the small particle size, toner should not be vacuumed with a conventional home vacuum cleaner. Static discharge from charged toner particles theoretically may ignite dust in the vacuum cleaner bag or create a small explosion if sufficient toner is airborne. Toner particles are so fine that they are poorly filtered by household vacuum cleaner filter bags and can blow through the vacuum motor into the room. They can also cause overheating by clogging the motor filter and short circuit by their electric conductivity (carbon, iron) when they melt inside the motor.