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Emergent behaviours are often characterised by the recurrent and recognizable events observable in a system’s macro-scale environment, which result from simple local interactions between system components. Such interactions are governed by simple rule sets, which lead to complex higher-level global behaviour. Hence, engineering emergence is a necessarily subtle process subject to the impacts of system evolution with minor changes at micro-scale giving completely different outcomes, to those envisaged, at the global system level. Thus, to harness the self-organisation and emergence as a design principle for the build and management of assured and trusted systems requires a principled approach to specification and reasoning on emergence. In this paper the authors advocate the use of a formal approach/method to specify component interactions, system evolution, and runtime global states. The proposed method provides a formal specification for the engineering of known emergence and runtime deliberation on the emergence observable in the global system. A process is detailed where a model with a formal specification of emergent behaviour is given in the SLABS language. The implementation is endowed with an observer system that reasons on novel emergent features with a consequent update of the formal system model. The process is evaluated by means of a newly created simulation that shows these ideas in operation.