EIN (Employer Identification Number)

Apply at IRS.gov (Form SS-4) — free, instant online for US-based applicants with SSN/ITIN. Format: XX-XXXXXXX. Required for: corporations, partnerships, multi-member LLCs, employers, businesses required to file employment/excise tax returns, trusts/estates, non-profits, retirement plans. Sole proprietors with no employees can use SSN instead but EIN provides privacy (don't give SSN to vendors/clients). EIN replaces "Federal Tax ID" terminology in most contexts. Once issued, EIN is permanent — not reassigned even if business dissolves.