Title: Runtime Monitoring of Concurrent Systems using polyLARVA Abstract: Sequential monitoring tools such as Larva are impractical for monitoring highly concurrent systems such as online establishments handling hundreds of transactions a second — they lock valuable resources which may otherwise be used to serve valid user requests. In the context of an open-source e-commerce system, we discuss design issues involved in allowing monitors to run concurrently while at the same time ensuring that they remain correct: free from race conditions and faithful to the properties they embody.