With the recent callup of military reserves, the question of employers' legal obligations to those employees is a common one.

Attorney Walter Liszka of Wessells & Pautsch, writing in the Mid-West Truckers Assn. magazine, notes that the Uniform Services Employment and Reemployment Rights Act of 1994, USERRA, covers all civilian employers, no matter how small.
The law says absences to perform any duty in the military service, whether voluntary or involuntary, are covered, including active duty as well as absences for training, weekend duty, summer camps, and fitness for duty examinations.
Employees are eligible to take military leave if they or an appropriate military officer gives the employer advance written or oral notice. No notice is required if doing so is impossible because of military necessity. An employer cannot require written proof of the need to take military leave. Only if the combined length of an employee's prior military leave is more than five years may future leaves be denied.
USERRA does not require pay during military leaves. However, if a company policy has established that the company will voluntarily pay reserves the difference between their regular wage and the military pay, then payment will be required and enforced.
Employees on military leave are entitled to participate in benefits not based on seniority, such as year-end bonuses, insurance coverage, accrual of sick or vacation days that is available to other employees on leave of absence.
Employers are free to fill vacancies left by employees on military leave, but a returning service member is entitles to reemployment in the position he or she left, regardless of whether another employee is occupying that position. Employees who have taken military leave generally have the right to return to their jobs without loss of seniority or benefits, but this part of the law is extremely complicated.
All states also have some law regulating military service and/or leave. If the federal and state laws differ, the federal law supercedes when state law offers a lower level of rights than USERRA. If the state law provides greater rights, the state law is the one that applies.