A comprehensive guide to using Monolog 3 effectively in PHP applications. Covers structured JSON logging, log levels, built-in and custom processors for context enrichment, key handlers (StreamHandler, RotatingFileHandler, FingersCrossedHandler), exception logging with chained causes, and two approaches to OpenTelemetry integration: zero-code PSR-3 auto-instrumentation and the explicit OTel Monolog handler. Concludes with guidance on centralizing logs in an OTLP-compatible observability platform for correlation with traces and metrics.