Klarna uses machine learning models to detect fraud in online transactions. They generate features from transactional data, customer profiles, merchants, and social graphs. Imbalanced class distributions are addressed using oversampling and undersampling techniques. Models include supervised algorithms like XGBoost and LightGBM as well as anomaly detection and ensemble meta models. Models are trained on AWS Sagemaker and deployed via Docker containers on Kubernetes clusters, which currently handle over 200 models. Key metrics include precision-recall curves and cost-based metrics to optimize for business goals. Production systems are monitored for performance, features, model metrics, and estimated business impacts.