As temperatures climb, millions of children all over the United States pack their bags each year and head to sleepaway camps ranging from the rustic to the super-luxurious. But it was not always so. Until the late 19th century, summers were for children to work or play in unstructured environments: Tag, hide-and-seek or hanging around adult work environments returning to their families at night. Since compulsory education laws only came about in the 1880s, sometimes these activities weren’t exclusive to the summer season.

Yet as the United States industrialized and both native-born transplants and immigrants flocked to crowded Northeastern cities, moral reformers and educators panicked. A new generation of children, especially boys, they fretted, were missing out on the character-building, health-promoting experiences of hardy rural life: some even mentioned the peril of “dying of indoor-ness.”

A stint at summer camp–surrounded by nature and engaged in hard work and healthy play, all under the guidance of counselors who modeled moral uprightness–was thought to be the perfect solution. Either genuinely bucolic or painstakingly constructed to suit romantic ideas of what a rural encampment should look like–imagine log cabins or a facility catering to white children featuring Native American décor–camps exemplified a “manufactured wilderness,” as historian Abigail Ayres Van Slyck described. This new social institution was soon embraced by many educators, philanthropists, and health professionals alike.

These early summer camps targeted middle-class, urban boys who it was feared were being “mollycoddled” by overbearing mothers and female teachers in the overly “feminized” realms of home and school. They needed a dose of savagery, common opinion held, lest they become “sissified.” Yet this middle-class project promoted an idea that reformers had first piloted with the working classes: in the 1850s, New York City’s Children’s Aid Society had shuttled “street rats” westward to be adopted by Christian farm families who were believed to be their last best hope for salvation from a life of poverty and vice.

This “uplift” mission also energized the first generation of summer camp boosters, who like many Progressive reformers believed the thousands of Eastern and Southern European children crowded into tenements with their foreign parents and unfamiliar customs would otherwise spent the summer loafing on the hot city streets, hardly learning the virtues or practices of American citizenship.

Camp wasn’t just for boys. By World War I, girls went to camp too. Learning to cook, sew, and otherwise prepare for motherhood was a standard part of cultivating virtuous young women who would resist the temptations of the “New Woman” who wore short skirts, smoked, and embraced her sexuality. Yet even while upholding traditional feminine ideals, these camps also created a rare opportunity for girls to leave home for an extended period, and many left feeling newly confident and independent. Interestingly, by the 1920s and 30s, many marginalized groups envisioned summer camp as an antidote to, rather than an engine of, Americanization.

African Americans excluded from segregated camps established their own institutions, like Massachusetts’ Camp Atwater (1921), devoted to provide recreational, social networking, and cultural opportunities to a growing black middle class. Jews, Christians and even socialists, all established summer camps to promote spiritual, cultural and political solidarity. Camp was catching on. By the 1930s, it was considered such an important rite of passage that New Deal money expanded philanthropic coffers to finance children’s summer sojourns even during the Depression.

During World War II, camp began to take on its modern form. Rather than prepare kids for adulthood, camps aimed to prolong and protect childhood innocence. Camps became places where children could explore individual passions–the arts, sports or outdoor life–in contrast to the presumed rigidity of school curricula, watchful gaze of doting parents, and the acculturating ethos that gave rise to camps in the first place. A professionalized camping industry that now works with private families, philanthropists and public agencies has expanded to create this expansive landscape.

Diverse as the American summer camp scene is, the fact that parents across social classes aspire to provide children with the experience suggests a shared assumption: many parents today agree kids should have free time in the summer to develop independently from their families, and are willing to pay for it. Perhaps the surest sign of camp’s cultural permanence is the recent rise of a cottage industry of adult summer camps where grownups leave behind technology and responsibility to connect with their inner child.