Caslon is the name given to serif typefaces designed by William Caslon I in London, or inspired by his work.

Caslon worked as an engraver of punches, the masters used to stamp the moulds or matrices used to cast metal type. He worked in the tradition of what is now called old-style serif letter design, that produced letters with a relatively organic structure resembling handwriting with a pen. Caslon established a tradition of engraving type in London, which previously had not been common, and was influenced by the imported Dutch Baroque typefaces that were popular in England at the time. His typefaces established a strong reputation for their quality and their attractive appearance, suitable for extended passages of text.

The letterforms of Caslon's roman, or upright type include an "A" with a concave hollow at top left and a "G" without a downwards-pointing spur at bottom right. The sides of the "M" are straight. The "W" has three terminals at the top and the "b" has a small tapered stroke ending at the bottom left. The "a" has a slight ball terminal. Ascenders and descenders are relatively short and the level of stroke contrast is modest in body text sizes. In italic, Caslon's "h" folds inwards and the "A" is sharply slanted. The "Q", "T", "v", "w" and "z" all have flourishes or swashes in the original design, something not all revivals follow. The italic "J" has a crossbar, and a rotated casting was used by Caslon in many sizes on his specimens to form the pound sign. However, Caslon created different designs of letters at different sizes: his larger sizes follow the lead of a type he sold cut in the previous century by Joseph Moxon, with more fine detail and sharper contrast in stroke weight, in the "Dutch taste" style. Caslon's larger-size roman fonts have two serifs on the "C", while his smaller-size versions have one half-arrow serif only at top right.

Caslon's typefaces were popular in his lifetime and beyond, and after a brief period of eclipse in the early nineteenth century, they returned to popularity, particularly for setting printed body text and books. Many revivals exist, with varying faithfulness to Caslon's original design. Modern Caslon revivals also often add features such as a matching boldface and "lining" numbers at the height of capital letters, neither of which were used in Caslon's time. William Berkson, designer of a revival of Caslon, describes Caslon in body text as "comfortable and inviting".

Caslon began his career in London as an apprentice engraver of ornamental designs on firearms and other metalwork. According to printer and historian John Nichols, the main source on Caslon's life, the accuracy of his work came to the attention of prominent London printers, who advanced him money to carve steel punches for printing, first for foreign languages and then, as his reputation developed, for the Latin alphabet. Punchcutting was a difficult technique and many of the techniques used were kept secret by punchcutters or passed on from father to son. Caslon would later follow this practice, according to Nichols teaching his son his methods privately while locked in a room where nobody could watch them. As British printers had little success or experience of making their own types, they were forced to use equipment bought from the Netherlands, or France, and Caslon's types are therefore clearly influenced by the popular Dutch typefaces of his period. James Mosley summarises his early work: "Caslon's pica ... was based very closely indeed on a pica roman and italic that appears on the specimen sheet of the widow of the Amsterdam printer Dirck Voskens, c.1695, and which Bowyer had used for some years. Caslon's pica replaces it in his printing from 1725…Caslon's Great Primer roman, first used in 1728, a type that was much admired in the twentieth century, is clearly related to the Text Romeyn of Voskens, a type of the early seventeenth century used by several London printers and now attributed to the punch-cutter Nicolas Briot of Gouda." Mosley also describes several other Caslon faces as "intelligent adaptations" of the Voskens Pica.

Caslon's type rapidly built up a reputation for workmanship, being described by Henry Newman in 1733 as "the work of that Artist who seems to aspire to outvying all the Workmen in his way in Europe, so that our Printers send no more to Holland for the Elzevir and other Letters which they formerly valued themselves much." Mosley describes Caslon's Long Primer No. 1 type as "type with generous proportions and it was normally cast with letter-spacing that was not too tight, characteristics that are needed in types on a small body. And yet it is so soundly made that words that are set in it keep their shape and are comfortably readable...It is a type that works best in the narrow measure of a two-column page or in quite modest octavos." Caslon sold a French Canon face he did not engrave that may to have been the work of Joseph Moxon with some modifications, and his larger-size faces follow this high-contrast model. He publicised his type through contributing a specimen sheet to Chambers' Cyclopedia, which has often been cut out by antiquarian book dealers and sold separately.

Compared to the more delicate, stylised and experimental "transitional" typefaces gaining ground in mainland Europe during Caslon's life, notably the romain du roi type of the previous century, the work of Pierre-Simon Fournier in Paris, Fleischmann in Amsterdam and the Baskerville type of John Baskerville in Birmingham that appeared towards the end of Caslon's career, Caslon's type was quite conservative. Johnson notes that his 1764 specimen "might have been produced a hundred years earlier". Stanley Morison described Caslon's type as "a happy archaism".

While not used extensively in Europe, Caslon types were distributed throughout the British Empire, including British North America, where they were used on the printing of the U.S. Declaration of Independence. After William Caslon I’s death, the use of his types diminished, but had a revival between 1840–80 as a part of the British Arts and Crafts movement.