It is widely recognized that research regarding complexity in projects has been inspired by Baccarini through his publication in 1996. Since then, research on this topic has been developing to other areas such as healthcare, IT, military, manufacturing, engineering, construction, and more. From theory point of view, the topic has also been researched in relation to various theories such as organization theory, PMI (Project Management Institute) view, system theory, complexity theory, among others. Interestingly, there is currently still no agreement among the researchers regarding the definition of complexity itself. Despite of this fact, studying complexity is important and will potentially improve project management practices, including in brownfield projects, where research has not much been pursued. Brownfield projects are important in securing sustained energy supply, as well as in realizing the transformation within the energy industry. This master’s thesis explores complexity in large oil and gas modification projects and aims to classify the complexity encountered by the individuals participating in such projects. A new framework for classifying project complexity is proposed by implementing grounded theory methodology over the data gathered through interviews and observations.