Monitoring tools allow us to keep an eye on our systems, applications, and micro-services, enabling alerting when things might go wrong, when they do go wrong, and where they might have gone wrong. API monitoring tools should provide testing, analytics/graphs, alerts, and integrations with CI/CD pipelines, allowing for efficient diagnosis of issues and ensuring clean deployments. Key metrics include time, latency, size, endpoint, arguments, environment, success, HTTP status code, location, and collecting these data points puts the API's activity in control. The best API monitoring tools include RapidAPI, Postman, Uptrends, Loggly, API Fortress, and AlertSite, each offering unique features such as functional testing, analytics, alerts, and integrations with CI/CD pipelines.