A dress code is a set of rules, often written, with regard to what clothing groups of people must wear. Dress codes are created out of social perceptions and norms, and vary based on purpose, circumstances, and occasions. Different societies and cultures are likely to have different dress codes, Western dress codes being a prominent example.

Dress codes are symbolic indications of different social ideas, including social class, cultural identity, attitude towards comfort, tradition, and political or religious affiliations. Dress code also allows individuals to read others' behavior as good, or bad by the way they express themselves with their choice of apparel.

From the seventh through the ninth centuries, the European royalty and nobility used a dress code to differentiate themselves from other people. All classes generally wore the same clothing, although distinctions among the social hierarchy began to become more noticeable through ornamented garments. Common pieces of clothing worn by peasants and the working class included plain tunics, cloaks, jackets, pants, and shoes. According to rank, embellishments adorned the collar of the tunic, waist or border. Examples of these decorations included, as James Planché states, "gold and silver chains and crosses, bracelets of gold, silver or ivory, golden and jeweled belts, strings of amber and other beads, rings, brooches, [and] buckles". The nobility tended to wear longer tunics than the lower social classes.

While dress codes of modern-day Europeans are less strict, there are some exceptions. It is possible to ban certain types of clothing in the workplace, as exemplified by the European Court of Justice’s verdict that "a ban on Islamic headscarves at work can be lawful."

The indigenous peoples of the Pacific Northwest Coast had a complex social hierarchy that consisted of slaves, commoners, and nobles, with dress codes indicating these social distinctions. John R. Jewitt, an Englishman who wrote a memoir about his years as a captive of the Nuu-chah-nulth people in 1802-1805, describes how, after some time living there, Maquinna and the chiefs decided that he must now be "considered one of them, and conform to their customs". Jewitt resented the imposition of this dress code, finding the loose untailored garments very cold, and attributed to them a subsequent illness of which he almost died. He was not allowed to cut his hair and had to paint his face and body as a Nootka would.

In the early 20th century, informal wear was the norm across many social settings, including workplaces, restaurants, travel, and movie theaters. In the 1950s, casual wear became prominent in many of these settings, but informal wear remained dominant in workplaces and churches. Beginning in the 1980s, technology companies in Silicon Valley developed the business casual dress code, which was part of a broader organizational culture of emphasizing efficiency over propriety. Today, casual wear is the norm in the tech industry, exemplified by tech executives such as Steve Jobs and Mark Zuckerberg.

In North American high schools, fashion for girls began to be more revealing in the late twentieth century, including clothing such as low-rise jeans, revealing tops, miniskirts, and spaghetti straps. With these new styles appearing in schools, dress codes have in some cases become more rigorous as a result.

The dress codes in North American high schools typically resulted in tests that would determine if skirts or shorts were long enough. A common test would be used to measure the appropriate length of students' shorts/skirts. If a student's fingers extended past their clothing, then the clothing was considered a violation of the school dress code.