Aspect-Oriented Programming (AOP) promises the modularization of so-called crosscutting functionalities in large applications. Currently, almost all approaches to AOP provide means for the description of sequential aspects that are to be applied to a sequential base program. In particular, there is no formally-defined concurrent approach to AOP, with the result that coordination issues between aspects and base programs as well as between aspects cannot precisely be investigated. This paper presents Concurrent Event-based AOP (CEAOP), which addresses this issue. Our contribution can be detailed as follows. First, we formally define a model for concurrent aspects which extends the sequential Event-based AOP approach. The definition is given as a translation into concurrent specifications using Finite Sequential Processes (FSP), thus enabling use of the Labelled Transition System Analyzer (LTSA) for formal property verification. Further, we show how to compose concurrent aspects using a set of general composition operators and sketch a Java prototype implementation for concurrent aspects we have realized.