Vibration monitoring has long been recognized as a reliable method for safeguarding compressor trains. Modern monitoring systems not only offer protection but also incorporate advanced features for diagnostics and condition assessment. This chapterDiagnostics explores howCondition monitoring condition monitoring technologies are applied to compressors, with particular emphasis on vibrationCondition monitoring monitoring as a key diagnostic and analytical tool. While other techniques are briefly discussedDiagnostics, they may hold equal significance within a comprehensive condition monitoring strategy. The chapter presents a high-level overview of this expansive subject, intentionally omitting detailed and specialized content for the sake of brevity.