Reinforcement Learning has moved beyond Atari games to production deployments. Key applications: 1) Cloud resource optimization: Google uses RL to reduce cooling energy in data centers by 30%. OpenAI Gym-style environment: state = {temperature, workload, fan_speed}, action = {adjust_cooling_by}, reward = -energy_usage. 2) Ad bidding and pricing: RL agents optimize real-time bidding in ad exchanges, learning policy to maximize conversion value given budget constraints. Multi-armed bandit variants for simpler exploration. 3) Recommendation systems: RL treats recommendation as sequential decision-making problem, explicitly models long-term engagement vs short-term click optimization. YouTube uses RL to avoid promoting low-quality viral content. 4) Supply chain optimization: inventory management RL trained on historical demand, learns order policies that reduce stockouts and holding costs simultaneously. Libraries: Stable Baselines 3 for standard algorithms (PPO, SAC, TD3, DQN). Ray RLlib for distributed training and production. Gymnasium (successor to OpenAI Gym) for environment standardization. Practical implementation: define environment class with observation_space, action_space, step(), reset(); wrap with gymnasium.Env. Start with PPO (Proximal Policy Optimization) - works well across diverse tasks. Reward shaping is critical and often the hardest part - dense rewards learn faster than sparse.