Abstract: In order to fulfil sets of similar user requirements within a specific application domain, one typically uses software product line engineering. In this paper, we investigate the nature of implementations of concerns, specific to software product line engineering. Based on these investigations, we present an approach that allows a modular specification and composition of concerns, with the purpose of constructing concrete variants of a software product line. The approach uses concepts from generic and aspect-oriented programming, and adapts them to the requirements imposed by software product line engineering.