Yes, as long as it's reasonable and doesn't discriminate. For example, many employers require employees to wear professional clothing, impose certain rules based on safety (such as no dangling jewelry), or have a uniform requirement. Rules like these are fine, as long you are open to an employee's request for a reasonable accommodation. For example, an employee who uses a wheelchair may need a uniform made out of softer material, to avoid discomfort and chafing. Or, an employee whose religious beliefs prohibit cutting his hair may ask to be allowed to wear his hair in a neat ponytail or under a head wrap rather than cutting it short. For more information on how to avoid discrimination when imposing dress codes, see Nolo's article Dress Codes and Grooming Codes in the Workplace.