Complexity has been identified, in both literature and practice, as a critical project characteristic that needs to be managed to improve project delivery. Many authors have argued that complexity accounts for some of the delays and cost overruns experienced on projects. Within the construction industry, there has been a shift from procuring many, but smaller projects by clients, to few but large contracts, in response to the demand for more integration and one-stop responsibility. The characteristics of such projects have been found to be interdependent, non-linear, dynamic and uncertain. Project management approaches to managing complex projects, have been based on classical Newtonian principles which are linear, mechanistic and reductionist in nature, but do not reflect accurately the reality of delivering such projects in multiproject environments. As yet, no study has conceptualised, operationalized nor tested a model for managing complex construction projects in a multi-project environment by the contractor. This study attempts to fill this gap. Based on a conceptual model, that provided understanding on the actuality of complex projects delivery in multi-project environments, a theoretical framework, based on socio-technical systems, was used to serve as a lens for empirical analysis for the study. Five complex construction projects (being delivered by contractors as part of their multi-project portfolio) were explored. The cases were integrated procurement types, in the form of Design and Build or PFI's. Case-study protocol was used to provide guidance for collecting data, and the framework for investigation and analysis.