Walden (/ˈwɔːldən/; first published as Walden; or, Life in the Woods) is an 1854 book by American transcendentalist writer Henry David Thoreau. The text is a reflection upon the author's simple living in natural surroundings. The work is part personal declaration of independence, social experiment, voyage of spiritual discovery, satire, and—to some degree—a manual for self-reliance.

Walden details Thoreau's experiences over the course of two years, two months, and two days in a cabin he built near Walden Pond amidst woodland owned by his friend and mentor Ralph Waldo Emerson, near Concord, Massachusetts.

Thoreau makes precise scientific observations of nature as well as metaphorical and poetic uses of natural phenomena. He identifies many plants and animals by both their popular and scientific names, records in detail the color and clarity of different bodies of water, precisely dates and describes the freezing and thawing of the pond, and recounts his experiments to measure the depth and shape of the bottom of the supposedly "bottomless" Walden Pond.

There has been much speculation as to why Thoreau went to live at the pond in the first place. E. B. White stated on this note, "Henry went forth to battle when he took to the woods, and Walden is the report of a man torn by two powerful and opposing drives—the desire to enjoy the world and the urge to set the world straight", while Leo Marx noted that Thoreau's stay at Walden Pond was an experiment based on his teacher Emerson's "method and of nature" and that it was a "report of an experiment in transcendental pastoralism".

Others have assumed Thoreau's intention during his time at Walden Pond was "to conduct an experiment: Could he survive, possibly even thrive, by stripping away all superfluous luxuries, living a plain, simple life in radically reduced conditions?" He thought of it as an experiment in "home economics". Although Thoreau went to Walden to escape what he considered "over-civilization", and in search of the "raw" and "savage delight" of the wilderness, he also spent considerable amounts of his time reading and writing.

Thoreau used his time at Walden Pond (July 4, 1845 – September 6, 1847) to write his first book, A Week on the Concord and Merrimack Rivers (1849). The experience later inspired Walden, in which Thoreau compresses the time into a single calendar year and uses passages of four seasons to symbolize human development.

By immersing himself in nature, Thoreau hoped to gain a more objective understanding of society through introspection. Simple living and self-sufficiency were Thoreau's other goals. The whole project was inspired by transcendentalist philosophy, a central theme of the American Romantic Period.

I went to the woods because I wished to live deliberately, to front only the essential facts of life, and see if I could not learn what it had to teach, and not, when I came to die, discover that I had not lived. I did not wish to live what was not life, living is so dear; nor did I wish to practice resignation, unless it was quite necessary. I wanted to live deep and suck out all the marrow of life, to live so sturdily and Spartan-like as to put to rout all that was not life, to cut a broad swath and shave close, to drive life into a corner, and reduce it to its lowest terms, and, if it proved to be mean, why then to get the whole and genuine meanness of it, and publish its meanness to the world; or if it were sublime, to know it by experience, and be able to give a true account of it in my next excursion.