An ignition interlock device (IID) is a breath-alcohol sensor wired into a vehicle's ignition. The driver must blow a sample below the threshold (typically 0.025% BAC) before the engine will start, and the device requires periodic rolling re-tests while driving.
In Missouri the interlock is often a choice rather than a condition, and choosing it early helps. On a first alcohol-related contact, RSMo § 302.525.2(1) gives you either a 30-day no-driving suspension followed by a 60-day restricted privilege, or, if you install a certified interlock, no suspension period at all and a 90-day restricted privilege instead. Refusal cases and repeat offenders face longer and mandatory interlock requirements.
Illinois requires an IID through the Monitoring Device Driving Permit (MDDP) for first-time DUI defendants during the Statutory Summary Suspension. Repeat offenders need an IID through the Restricted Driving Permit issued by the Secretary of State.
IID violations (failed tests, missed rolling re-tests, tampering) get reported automatically to the state and can extend the underlying suspension or trigger new criminal charges.