A blackboard or a chalkboard is a reusable writing surface on which text or drawings are made with sticks of calcium sulphate or calcium carbonate, better known as chalk.

Blackboards were originally made of smooth, thin sheets of black or dark grey slate stone.

A blackboard can simply be a board painted with a dark matte paint (usually black, occasionally dark green). Matte black plastic sign material (known as closed-cell PVC foamboard) is also used to create custom chalkboard art. Blackboards on an A-frame are used by restaurants and bars to advertise daily specials. Adhesive chalkboard surface is also available in stores as rolls of textured black plastic shelf covering, which is applied to the desired wall, door or other surface.

A more modern variation consists of a coiled sheet of plastic drawn across two parallel rollers, which can be scrolled to create additional writing space while saving what has been written. The highest grade blackboards are made of porcelain-enameled steel (black, green, blue or sometimes other colours). Porcelain is very hard wearing, and blackboards made of porcelain usually last 10–20 years in intensive use.

Lecture theatres may contain a number of blackboards in a grid arrangement. The lecturer then moves boards into reach for writing and then moves them out of reach, allowing a large amount of material to be shown simultaneously.

Chalk sticks are produced in white and in various colours, especially for use with blackboards. White chalk sticks are made mainly from calcium carbonate derived from mineral chalk or limestone, while coloured chalk sticks are made from calcium sulphate in its dihydrate form, CaSO₄·2H₂O, derived from gypsum. Chalk sticks containing calcium carbonate typically contain 40–60% of CaCO₃ (calcite).

Chalk dust can aggravate respiratory conditions such as asthma and allergies, according to the American Academy of Allergy, Asthma and Immunology (AAAAI). The dust can also damage or interfere with dust-sensitive equipment, including computers. Chalk sticks wear down quickly and are prone to breaking unless held in a protective chalk holder. Over time, blackboards may experience “ghosting,” in which residual pigment from chalk or chalk markers absorbs into the surface and cannot be fully erased.

The scratching of fingernails on a blackboard, as well as other pointed, especially metal objects against blackboards, produces a sound that is well known for being extremely irritating to most people. According to a study run by Michael Oehler, a professor at the University of Cologne, Germany, humans are "predisposed to detest" the sound of nails on a blackboard. The findings of the study were presented at the Acoustical Society of America conference and support earlier findings from a 1986 study by Vanderbilt psychologist Randolph Blake and two colleagues found that the sound of nails on a chalkboard annoyed people even when the high-pitch frequencies were removed. The study earned Blake a 2006 Ig Nobel Prize.