In the context of remote sensing, validation refers to the process of assessing the uncertainty of higher level, satellite sensor derived products by analytical comparison to reference data, which is presumed to represent the true value of an attribute. Biophysical products characterise and map biotic and abiotic factors that influence the survival, development and evolution of organisms within the environment. Naturally, validation is an essential component of any earth observation program, since it enables the independent verification of the physical measurements obtained by a sensor as well as any derived products. After presenting some relevant definitions, this chapter draws on international and national validation campaigns to summarize some of the major components involved when using ground-reference data to validate biophysical products derived through Earth Observation (EO) data. These include site selection, site extent, and sampling design. Major Australian and international validation campaigns are exemplified for Leaf Area Index (LAI) and Foliage Projective Cover (FPC) vegetation products. The process of up-scaling, which enables the validation of coarse resolution products via the comparison of measurements made at various scales (i.e., ground-based, intermediate-airborne) is also reviewed. The chapter concludes with a brief section on alternative validation methods.