A modal dialog is an overlay that appears on top of your page, typically with a dimmed background, requiring the user to interact with it (close it, fill it out, or make a choice) before they can return to the main content. It's the most interruptive UI pattern available, which is exactly why it works — and exactly why it's dangerous.
The data on modals is contradictory because context is everything. Exit-intent modals with a genuine offer can recover 5-15% of abandoning visitors — that's real revenue from people who were leaving anyway. But entry modals that appear before users have even read your headline? They average a 40% close rate within 2 seconds, meaning nearly half your visitors' first interaction with your brand is clicking an X button. That's a terrible first impression.
Rules for modals that convert without annoying
Timing matters more than the offer. Don't show a modal until the visitor has demonstrated intent — scrolled past 50% of the page, spent 30+ seconds, or triggered exit intent. The same offer converts significantly better when it appears to an engaged user versus an arriving one.
Always make the close action obvious and easy — a visible X button AND clicking outside the modal should both dismiss it. Mobile modals need extra care: they must be scrollable if content is long, the close button must be reachable by thumb, and they should never trigger on scroll (too easy to invoke accidentally). Google penalizes intrusive interstitials on mobile, so SEO is another reason for restraint.