John Muir (1838 – 1914) was a Scottish-American naturalist, author, and early advocate of preservation of wilderness in the United States. His letters, essays, and books telling of his adventures in nature, especially in the Sierra Nevada mountains of California, have been read by millions. His activism helped to preserve the Yosemite Valley, Sequoia National Park and other wilderness areas. The Sierra Club, which he founded, is now one of the most important conservation organizations in the United States. One of the best-known hiking trails in the U.S., the 211-mile (340 km) John Muir Trail, was named in his honor. Other such places include Muir Woods National Monument, Muir Beach, John Muir College, Mount Muir, Camp Muir and Muir Glacier.

In his later life, Muir devoted most of his time to the preservation of the Western forests. He petitioned the U.S. Congress for the National Park bill that was passed in 1890, establishing Yosemite and Sequoia National Parks. The spiritual quality and enthusiasm toward nature expressed in his writings inspired readers, including presidents and congressmen, to take action to help preserve large nature areas. He is today referred to as the "Father of the National Parks" and the National Park Service has produced a short documentary about his life.

Muir's biographer, Steven J. Holmes, believes that Muir has become "one of the patron saints of twentieth-century American environmental activity," both political and recreational. As a result, his writings are commonly discussed in books and journals, and he is often quoted by nature photographers such as Ansel Adams. "Muir has profoundly shaped the very categories through which Americans understand and envision their relationships with the natural world," writes Holmes. Muir was noted for being an ecological thinker, political spokesman, and religious prophet, whose writings became a personal guide into nature for countless individuals, making his name "almost ubiquitous" in the modern environmental consciousness. According to author William Anderson, Muir exemplified "the archetype of our oneness with the earth".

Muir was extremely fond of Henry David Thoreau and was probably influenced more by him than even Ralph Waldo Emerson. Muir often referred to himself as a "disciple" of Thoreau. He was also heavily influenced by fellow naturalist John Burroughs.

During his lifetime John Muir published over 300 articles and 12 books. He co-founded the Sierra Club, which helped establish a number of national parks after he died and today has over 1.3 million members. Author Gretel Ehrlich states that as a "dreamer and activist, his eloquent words changed the way Americans saw their mountains, forests, seashores, and deserts." He not only led the efforts to protect forest areas and have some designated as national parks, but his writings gave readers a conception of the relationship between "human culture and wild nature as one of humility and respect for all life," writes author Thurman Wilkins.

His philosophy exalted wild nature over human culture and civilization. Turner describes him as "a man who in his singular way rediscovered America. . . . an American pioneer, an American hero." Wilkins adds that a primary aim of Muir’s nature philosophy was to challenge mankind’s "enormous conceit," and in so doing, he moved beyond the Transcendentalism of Emerson and Thoreau to a "biocentric perspective on the world."

In the months after his death, many who knew Muir closely wrote about his influences.

John Muir was one of our first and finest writers on the wilderness of the American West. Part of Muir's attractiveness to modern readers is the fact that he was an activist. He not only explored the West and wrote about its beauties-- he fought for their preservation. His successes dot the landscape in all the natural features that bear his name: forests, lakes, trails, glaciers. Here collected are some of his finest wilderness essays, ranging from Alaska to Yellowstone, from Oregon to the Range of Light-- the High Sierra. This series celebrates the tradition of literary naturalists-- writers who embrace the natural world as the setting for some of our most euphoric and serious experiences. Their literary terrain maps the intimate connections between the human and natural worlds, a subject defined by Mary Austin in 1920 as "a third thing... the sum of what passed between me and the Land." Literary naturalists transcend political boundaries, social concerns, and historical milieus; they speak for what Henry Beston called the "other nations" of the planet. Their message acquires more weight and urgency as wild places become increasingly scarce. This series, then, celebrates both a wonderful body of work and a fundamental truth: that nature counts as a model, a guide to how we can live in the world.