[9] When he himself was called to the orthopaedic workshop of the Friedrichsheiner Klinik in Frankfurt am Main – and therefore did not have to do military service – he relocated his business activities.

[9] With this invention, Konrad Birkenstock had followed the ideas of the shoe reform, which, in keeping with the times, rethought the human body.

[12][13][14] In the 1920s, Birkenstock insoles were already being supplied to Austria, France, Denmark, Czechoslovakia, Italy, Luxembourg, Belgium, Norway, the Netherlands, Sweden and Switzerland.

He visited specialist shoe stores and shoemakers and explained to them the use and effect of the "Birkenstock system", as the combination of a fully plastic last and flexible footbed was soon called.

[9] Carl Birkenstock took the idea of foot health even further than his father and limited sales of his insoles to specialist retailers and shoemakers who had successfully completed his training course.

Birkenstock GmbH in Steinhude im Meer - employed 13 people during the National Socialist era, including the owners, 6 of whom were drafted into the Wehrmacht.

[2][9][18] Birkenstock therefore did not do any business with the NSDAP, other party organizations or the Wehrmacht itself; he did not have any materials or deposits checked on the Schuhprüfstrecke of the Sachsenhausen concentration camp.

[21] After 1945, Carl Birkenstock continued to pursue his idea of his handmade healthy footwear at his new headquarters in Bad Honnef near Bonn.

[20] During this time, the still very small company earned money through the successful sale of the "Blue Footbed" orthopaedic insole.

While his grandfather Konrad and father Carl Birkenstock worked all their lives on producing orthopaedic insoles that were as individualized as possible for the wearer, the young Karl Birkenstock succeeded in making healthy footwear mass-produced in 1962 with the invention of a standardized, permanently installed, anatomically shaped insole.

The shoe was constructed so that the wearer would have to grip their toes to keep the sandals on; this resulted in toning the calf muscle,[23] which became quite useful to athletes, especially among gymnasts.

[20] In 1966, Margot Fraser, a German-American dressmaker who resided in Santa Cruz, California, decided to travel back to Germany to visit a spa in Bavaria, where she was recommended Madrids to help with a foot ailment caused by tight shoes.

[23][26][27] Due to Fraser's relief from her foot condition and her enthusiasm for the sandals, Birkenstocks were introduced in the United States—though some hurdles stood in the way of their eventual acceptance by American buyers.

In the United States, Birkenstocks were first popular among young men and later on among flower children, a group traditionally associated with American liberalism.

Since the 1960s, Birkenstock sandals had become popular in the care professions and in the alternative and peace movement, and then also in "middle-class" households as slippers and leisure shoes.

The handover took the form of a multi-brand strategy, with Alexander, Christian and Stephan Birkenstock each responsible for their own brands and companies.

[39] Each of them had different ideas for the company's future and ended up creating multiple competing product lines and subsidiaries.

[30] As a result, the sandals also became increasingly popular in the so-called mainstream, until the multi-brand strategy gradually eroded customer interest.

[41] Birkenstocks saw another comeback among teens and young adults in the early 2010s, as well as in the late 2010s due in part to the VSCO girl trend.

In 2013, Stephan sold his stake to the two brothers; they recruited Oliver Reichert, an outsider, to consolidate various subsidiaries into Birkenstock Group and lead the company.

[23][44][45] In February 2021, the American-French investment company L Catterton and the family holding Financière Agache, the private equity firm backed by Bernard Arnault's LVMH, acquired a majority stake in the German sandal manufacturer Birkenstock.

[53][54] Besides Céline's creative director Phoebe Philo, many other designers have also created their version of the sandal, including Giambattista Valli and Givenchy.

[23] In 2019 Valentino collaborated with Birkenstock to create their version of the sandal featured at Men's Paris Fashion Week.

The shoe is made of the same materials as the original footbed, with the addition of a foam insert placed under the suede lining.