Mixture of Experts (MoE) has become one of the most important architectural innovations in modern large language models, enabling massive scale while keeping computational costs manageable. If you’ve wondered how cutting-edge 2025 models like OpenAI's GPT-5 and GPT-OSS-120B, Moonshot's trillion-parameter Kimi K2, or DeepSeek's V3.1 can have hundreds of billions or even trillions of parameters while still being practical to run, MoE is the secret sauce behind their efficiency.