Duolingo's backend leverages machine learning models for adaptive learning algorithms, including spaced repetition systems (SRS) that optimize when vocabulary and grammar concepts are reviewed for maximum retention. The premium Duolingo Max tier integrates OpenAI's GPT-4 to power two flagship AI features: Roleplay (open-ended AI conversation practice with a virtual character) and Explain My Answer (contextual, natural-language explanations for why an answer was right or wrong). The platform runs A/B experiments at massive scale across its user base to continuously improve lesson design, engagement mechanics, and learning outcomes. Mobile apps (iOS and Android) and a web interface are supported, with offline mode available for downloaded lessons.