Taxpayer Identification Numbers Explained: SSN, ITIN, and EIN

Overview

Taxpayer Identification Numbers (TINs) are the primary way the IRS and other federal agencies link tax returns, credits, payments, and identity records to the correct person or entity. The three TINs most readers will encounter are the Social Security Number (SSN), Individual Taxpayer Identification Number (ITIN), and Employer Identification Number (EIN). Each serves a distinct purpose, and using the wrong one — or omitting a TIN on required forms — can delay refunds, trigger backup withholding, or create additional verification steps.

In my 15+ years as a CPA helping individuals and business owners, most problems I see stem from confusion about which TIN to use and how to get one quickly when it’s required. This guide explains how each TIN works, who should apply, how to apply, practical timelines, common mistakes, and next steps if something goes wrong.

Note: This article is educational and not a substitute for personalized tax or legal advice. For case‑specific direction, consult a licensed tax professional.

How the three main TINs differ

• Social Security Number (SSN): Issued by the Social Security Administration (SSA) primarily to U.S. citizens and eligible residents. The SSN is used for Social Security benefits, wage reporting, and most individual tax filing. (SSA: Form SS‑5 and instructions at https://www.ssa.gov/forms/ss-5.pdf)

• Individual Taxpayer Identification Number (ITIN): Issued by the IRS to individuals who must file U.S. tax returns but are not eligible for an SSN. Typical users include certain nonresident aliens, foreign spouses and dependents, and others with U.S. tax filing obligations. Apply using IRS Form W‑7. (IRS: https://www.irs.gov/individuals/get-an-itin)

• Employer Identification Number (EIN): Issued by the IRS to businesses, trusts, estates, and certain other entities for reporting employment taxes, business income, and other tax responsibilities. Apply via IRS Form SS‑4; many applicants receive an EIN immediately when using the online application. (IRS: https://www.irs.gov/businesses/small-businesses-self-employed/apply-for-an-ein-online)

Who needs each TIN — practical examples

• You and your family: If you are a U.S. citizen or green‑card holder, you (and qualifying dependents) should have an SSN. Newborns and adopted children typically get an SSN to enable dependent tax credits and to be claimed on returns.

• Nonresident or foreign taxpayer with filing obligations: If you must file a federal tax return but can’t get an SSN, you need an ITIN. For example, a nonresident spouse filing jointly or a foreign national who earned U.S. source income often needs an ITIN.

• Business owner or employer: If you operate a business, hire employees, or need to file business tax forms, you generally need an EIN. Even sole proprietors can use an EIN to separate personal and business activity for banking and payroll.

Real client examples from my practice:

• A remote worker in Mexico contracted by a U.S. company needed an ITIN to file and claim allowable deductions; we filed Form W‑7 with certified supporting documents and completed the return.
• A client forming a multi‑member LLC obtained an EIN the same day via the IRS online system so they could open a business bank account and process payroll.

Application process and expected timelines

• SSN (SSA Form SS‑5): Apply in person at an SSA office or by mail with required identity and citizenship documents. Processing for a new SSN card typically takes 2–4 weeks once the SSA has acceptable documentation, though timelines vary by office. (SSA guidance: https://www.ssa.gov)

• ITIN (IRS Form W‑7): File Form W‑7 with your U.S. federal tax return (or a valid exception reason). You must submit original or certified copies of identity documents (passport, national ID, etc.). Processing can range from several weeks to a few months depending on IRS workload and whether you submit original documents or use an IRS‑authorized acceptance agent. (IRS: https://www.irs.gov/individuals/get-an-itin)

• EIN (IRS Form SS‑4): Apply online, by fax, or by mail. The online EIN assistant issues the number immediately for qualifying applicants with a principal business in the U.S. Fax response times vary; mailed requests take longer. (IRS: https://www.irs.gov/businesses/small-businesses-self-employed/apply-for-an-ein-online)

Documents you’ll commonly need

• SSN: Proof of identity and citizenship or immigration status (e.g., U.S. birth certificate, passport, Form I‑94, or Permanent Resident Card). SSA details are authoritative. (SSA: https://www.ssa.gov)
• ITIN: Form W‑7 plus passport is the most common single document that proves both identity and foreign status; other documents may be acceptable. Consider using an IRS acceptance agent to avoid mailing originals. (IRS: https://www.irs.gov/individuals/get-an-itin)
• EIN: Basic business information and responsible party identification (name, SSN or ITIN, address). If you don’t have an SSN/ITIN, the EIN application still accepts foreign applicants under certain conditions; read the SS‑4 instructions. (IRS: https://www.irs.gov/forms-pubs/about-form-ss-4)

Common pitfalls and how to avoid them

• Using the wrong TIN on forms: For example, listing an ITIN where an SSN is required (or vice versa) will trigger notices and may delay refunds. Always check form instructions before filing.

• Mailing original documents unnecessarily: If possible, use certified copies from the issuing agency or an IRS acceptance agent to avoid sending passports and other originals to the IRS.

• Assuming an ITIN equals work authorization: ITINs are only for tax reporting and do not authorize employment or grant immigration benefits.

• Not renewing an ITIN: ITINs can expire. The IRS explains which ITINs require renewal and the renewal process on its ITIN page. If an ITIN has expired, claims for refunds or credits can be delayed. (IRS: https://www.irs.gov/individuals/get-an-itin)

• Identity theft risk: Treat any TIN — especially SSNs — as highly sensitive. Limit sharing to trusted parties and verify requests before disclosing a TIN. The IRS and SSA both provide guidance on protecting your SSN and tax identity.

If you lose or suspect misuse of a TIN

• SSN misuse or identity theft: Contact the SSA for replacement cards and the IRS Identity Protection Specialized Unit if tax fraud is suspected. Also place fraud alerts with credit bureaus.

• EIN issues: If you lose your EIN letter, you can find the EIN on prior tax returns or call the IRS Business & Specialty Tax Line to request verification.

• ITIN problems: If an ITIN has expired or is lost, follow the IRS renewal steps in Form W‑7 instructions. Consider using an acceptance agent if you need help with documentation.

How TINs affect filing, refunds, and verification

TIN accuracy affects filing acceptance, refund timing, and identity verification. For example, incorrect or missing TINs commonly trigger IRS notices or identity checks that delay refunds. For more on how identity verification interacts with refund processing and what to expect when the IRS requests proof, see our guide on “How Identity Verification Affects Refund Processing.” (FinHelp: https://finhelp.io/glossary/how-identity-verification-affects-refund-processing/)

If you’re new to filing U.S. returns electronically, our step‑by‑step primer “How to File a Tax Return Electronically for the First Time” walks through using modern e‑file options and how TINs are used in the e‑file process. (FinHelp: https://finhelp.io/glossary/how-to-file-a-tax-return-electronically-for-the-first-time/)

Quick decision checklist

• Are you a U.S. citizen or permanent resident (green card)? Apply for or use an SSN.
• Are you not eligible for an SSN but need to file U.S. taxes? Apply for an ITIN with Form W‑7.
• Do you run a business, hire employees, or file business tax forms? Get an EIN with Form SS‑4.

Final tips from a CPA

1. Keep a secure copy of each TIN confirmation letter or card and store originals in a safe place.
2. When a TIN is required on a form, double‑check the number before submitting — transposition errors are common and can create costly follow‑ups.
3. If you need to send identity documents, use certified copies or an IRS acceptance agent to avoid mailing passports.
4. Use an EIN for business banking and hiring to separate personal SSN exposure from business operations.

Related internal reading

• Social Security Number (SSN): an in‑depth look at how SSNs are issued and protected (FinHelp: https://finhelp.io/glossary/social-security-number-ssn/)
• How Identity Verification Affects Refund Processing (FinHelp: https://finhelp.io/glossary/how-identity-verification-affects-refund-processing/)
• How to File a Tax Return Electronically for the First Time (FinHelp: https://finhelp.io/glossary/how-to-file-a-tax-return-electronically-for-the-first-time/)

Sources and authoritative references

• IRS — Apply for an EIN (Form SS‑4): https://www.irs.gov/businesses/small-businesses-self-employed/apply-for-an-ein-online
• IRS — ITIN information and Form W‑7: https://www.irs.gov/individuals/get-an-itin
• Social Security Administration — SSN application (Form SS‑5) and documentation: https://www.ssa.gov/forms/ss-5.pdf

Professional Disclaimer: This content is educational and general in nature. It does not provide individualized tax, legal or financial advice. For guidance tailored to your circumstances, consult a licensed tax professional or attorney.