Geometric documentation is one of the most important task of the Cultural Heritage (CH) conservation and management policies. 3D documentation, prior to any conservation and restoration works, is considered a basic pre-requisite for preserving, understanding, communicating and valorizing CH sites and objects (London Charter, 2009; Sevilla Principles, 2011). <br><br> 3D models have become the usual way of digitally preserving, communicating, explaining and disseminat ing cultural knowledge, as they have the capability of reproducing ancient states and behaviors. Using photo-realistic and accurate 3D models, the current conservation state can be shown and preserve for future generations. But despite the large request of 3D models in the CH field, there is no 3D documentation method which can properly satisfy all the areas with their requirements, therefore a fusion methodology (of data and sensors) is normally required and performed. The paper analyzes the fusion concept and levels as well as some merging approaches so far presented in the research community. While the paper will be necessarily incomplete due to space limitations, it will hopefully give an understanding on the actual methods of data fusion and clarify some open research issues.