Research has shown advantages an organization can obtain by implementing lean methodology. However, when implementing new philosophies like Lean Construction, there are always some challenges to overcome. Examples could be lack of basic knowledge about the theory/philosophy behind lean, lack of willingness to change, implementation process in an organization, lack of clear objectives/visions with the implementation and top management involvement. The authors of this paper have worked with implementation of lean principles in a Norwegian contractor company. The contractor is split into several sister-companies, acquired over the past 40 years. Some of the sister-companies have succeeded in implementing lean, while others have not succeeded well. The authors will evaluate how the organization have planned to implement lean in an efficient and sustainable way, and what factors that have affected the implementation. The research methodology used is case study, where the different sister-companies are the cases. Research from the case study of lean implementation will give a better understanding for similar cases. It will also address how to overcome challenges related to the implementation process in similar cases.