Every morning, more than 220,000 workers cross the Luxembourg border from Thionville, Metz, Arlon, Trier and dozens of small villages most people have never heard of. Here's what nobody tells you: a huge chunk of them are paying taxes in the wrong country, missing reimbursements they're legally owed, or panicking every December because they crossed the magic 34-day telework limit without realizing it. I've seen this mistake hundreds of times — a frontalier from Thionville accepts a remote-friendly contract, works from home 40 days, and suddenly France wants to tax part of their Luxembourg salary. The fix would have cost them nothing. The mistake costs them thousands. Cross-border work in the EU isn't really one system — it's a patchwork held together by Article 45 TFEU (free movement of workers), Regulation 883/2004 (which social-security system applies to you), and a stack of bilateral tax treaties that almost nobody reads. For 2026, the rules that matter most for frontaliers are simple on paper: you're taxed where you physically work, you contribute to social security in one country only (the A1 form proves it), and you can telework up to 34 days per year from your country of residence without breaking the tax arrangement. Cross 34 days and the tax authority back home gets a slice. Cross 25% of your annual working time from home and your social security can flip to your country of residence — which usually means much higher contributions. If you live in France and work in Luxembourg, you'll see roughly €2,400 net from a €3,000 gross salary, versus about €2,250 if the same job were taxed in France. That €150/month gap is exactly why 120,000 French residents commute daily. But the gap evaporates fast if you mess up the telework declaration, forget to claim the frontalier allowance, or pick the wrong commune for residence. This page exists because most of those mistakes are completely avoidable — once you know the rules.