In a major part of the history of orchestration timbre has been seen as a secondary parameter. This thesis is a study of how timbre can be used as primary parameter in orchestration. Through analysis of Boléro by Maurice Ravel, Lontano by Gÿorgy Ligeti and 48 Responses to Polymorphia: Overtones by Jonny Greenwood, different approaches to this use is presented. This includes how timbral knowledge can be used as base for creating compositional ideas and how other parameters as harmony, melody, form and texture can function as secondary parameters to manipulate the timbre. The thesis also contains a practical review and execution where theories and ideas are put into action with a composition for a string orchestra.