Decay is an incremental simulation game where players create atomic nuclei by launching protons and neutrons. The core loop involves configuring particle combinations, launching them to form nuclei, and earning in-game currency from successful experiments. This currency is then reinvested into laboratory upgrades and automation systems to increase earning potential.

The game allows players to discover hundreds of rare isotopes and observe real atomic decay chains. Players can upgrade their lab, expand capabilities, and automate multiple experiments in parallel. Racctuality, the developer, is committed to providing all future gameplay updates for free, with no paid gameplay DLC planned.