The notion of aspect looks promising for handling crosscutting concerns earlier in the software life-cycle, up from programming to design, analysis and even requirements. This paper proposes a technique to encapsulate and weave behaviors described as sequence diagrams in base models which contain a finite set of sequence diagrams. The weaving process is twophased. Firstly the detection phase searches parts in the base model. Secondly the composition phase builds the woven model by composing the advise in the base model for each detected part. The entire weaving process is automated and is implemented as model transformations within the Kermeta environment.