Guard & Reserve
Leave for military duty and return to the job, seniority, and pay you'd have had.

Pennsylvania National Guard photo by Brad Rhen, DVIDS (public domain).
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Open LES Tool→USERRA, the Uniformed Services Employment and Reemployment Rights Act, protects your civilian job when you leave it for military duty, including drill, annual training, schools, and mobilization. Done right, you come back to the job, and the seniority, pay, and benefits, you would have had if you never left.
The protections are strong, but they come with steps: give your employer notice, stay inside the time limits, and report back on time.
USERRA covers nearly all employers, large and small, and most kinds of military service. The headline protections:
What you are owed
The limits and duties
USERRA returns you to where you would have been, not just your old seat. That is the escalator principle, and it protects the raises and promotions you would have earned.
Source: U.S. Department of Labor (VETS); ESGR
If your military service lasts more than 30 days, you can keep your employer health plan for up to 24 months, though you may pay up to 102% of the full premium. For service of 30 days or less, your coverage continues as if you stayed at work. When you return, your plan cannot impose a waiting period or exclusion that would not have applied if you had never left.
Your deadline to report back or reapply depends on how long you served. Short stints mean reporting back almost immediately; longer tours give you more time to apply for reemployment. Hit the window, and your employer must reemploy you promptly.
Reporting back (general guide)
If your employer pushes back, ESGR offers free mediation, and the Department of Labor (VETS) enforces USERRA.
Does USERRA cover drill and annual training?
Yes. It covers voluntary and involuntary service, including drill, annual training, schools, and mobilization.
What is the escalator principle?
You return to the position, seniority, and pay you would have attained had you not left, not simply your former job.
Can I keep my employer health insurance?
For service over 30 days, you can continue it up to 24 months (you may pay up to 102% of the premium). For 30 days or less, it continues as if you were at work.
What if my employer will not reemploy me?
Contact ESGR for free mediation; the Department of Labor's VETS enforces USERRA, and you can pursue your rights at no cost.