To quantify strain, measurements may be benificial. This chapter outlines methods to obtain the principal stretchs from strain measurements from arbitrary outcrop surfaces. This starts with an outline of the major types of marker objects used for strain determination. The distiction of different strain types and analysis of strain and general 3D deformation follows. Volume changes due to 3D deformation are considered in the sixth paragraph. The following three paragraphs explain a selection of the most practical techniques available for strain analysis. Finally, the influence of principle stresses on finite strain estimations is considered.