Commonplace books (or commonplaces) are personal notebooks used to compile any information the owner finds interesting or useful. They can variously contain notes, proverbs, adages, aphorisms, maxims, recipes, quotes, letters, poems, tables of weights and measures, prayers, legal formulas, and other professional references. They have been kept from antiquity, and were kept particularly during the Renaissance and in the nineteenth century.

Entries are most often organized under systematic subject headings and differ functionally from journals or diaries, which are chronological and introspective.

"Commonplace" is a translation of the Latin term locus communis (from Greek tópos koinós, see literary topos) which means "a general or common place", such as a statement of proverbial wisdom. In this original sense, commonplace books were collections of such sayings, such as John Milton's example. "Commonplace book" is at times used with an expansive sense, referring to collections by an individual in one volume which have a common theme (e.g. ethics) or explores several themes. The term overlaps with aspects of the terms "anthology" or "mixed-manuscript" in these productions but most properly refers to a collection of sayings or excerpts by an individual, often collected under thematic headings.

Commonplaces are a separate genre of writing from diaries or travelogues. Commonplaces are used by readers, writers, students, and scholars as an aid for remembering useful concepts or facts; sometimes they were required of young women as evidence of their mastery of social roles and as demonstrations of the correctness of their upbringing. They became significant in Early Modern Europe. As a genre, commonplace books were generally private collections of information, but as the amount of information grew following the invention of movable type and printing became less expensive, some were published for the general public.

In 1685 the English Enlightenment philosopher John Locke wrote a treatise in French on commonplace books, translated into English in 1706 as A New Method of Making Common-Place-Books, "in which techniques for entering proverbs, quotations, ideas, speeches were formulated." Locke gave specific advice on how to arrange material by subject and category, using such key topics as love, politics, or religion. Following the publication of his work, publishers often printed empty commonplace books with space for headings and indices to be filled in by their users. An example is "Bell's Common-Place Book, Formed generally upon the Principles Recommended and Practised by Mr Locke" which was published by John Bell almost a century after Locke's treatise. A copy of this blank commonplace was used by Erasmus Darwin from 1776 to 1787, and it was later used by Charles Darwin who called it "the great book" when composing his grandfather's biography.

By the early eighteenth century, they had become an information management device in which a note-taker stored quotations, observations, and definitions. They were used in private households to collate ethical or informative texts, sometimes alongside recipes or medical formulae. For women, who were excluded from formal higher education, the commonplace book could be a repository of intellectual references. The gentlewoman Elizabeth Lyttelton kept one from the 1670s to 1713 and a typical example was published by Mrs Anna Jameson in 1855, including headings such as Ethical Fragments; Theological; Literature and Art.

Commonplace books were used by scientists and other thinkers in the same way that a database might now be used: Carl Linnaeus, for instance, used commonplacing techniques to invent and arrange the nomenclature of his Systema Naturae (which is the basis for the system used by scientists today).

The commonplace system of categorized note-keeping was not restricted to books. In the twentieth century, Henri de Lubac traveled with his notes in a sack. Erasmus of Rotterdam traveled with a chest of notes, including examples of well-written Latin that formed the basis of his Adagia. In De Copia his Method of Collecting Examples (Ratio collegendi exampla) advocated a hierarchical but ad hoc breakdown of topics: for example, the top-level might be Piety and Impiety, under Piety might come Gratitude, and under these headings one puts example texts. The commonplace proper would be some simple aphorism or moral, possibly several, that can be drawn from the example, such as The crowd loves and hates thoughtlessly.