Leaf Area Index (LAI) is typically defined as the total one-sided area of leaf tissues per unit of ground surface area. Utilizing this definition, LAI is a dimensionless unit which characterises the canopy of a given ecosystem (Breda, 2003). LAI and the fraction of absorbed photosynthetically active radiation (fAPAR) are two biophysical parameters that are closely related and often measured and validated in parallel in the field. fAPAR is defined as the fraction of photosynthetically active radiation (PAR) in the 400-700 nm wavelengths that is absorbed by a canopy and it can include over-storey, understory and ground cover elements (Gower et al., 1999; Fensholt et al., 2004). This chapter provides a basic review of LAI product validation, supplemented with information on the allied metric fAPAR. After presenting a brief introduction of these concepts (LAI and fAPAR), some of the major global LAI product validation programs are reviewed. This is followed by a discussion of different validation methods that can be used in the field and in situ sensors used to collect LAI and fAPAR measurements (e.g., Li-Cor LAI-2200, TRAC, AccuPar Ceptometer, Digital Hemispherical Photography or DHP and the Plant Canopy Imager CI-110). The chapter finishes by presenting a methodology that illustrates MODIS collection 5 LAI validation efforts in Australian vegetated ecosystems. Additional guidance on sampling designs that can be used for LAI validation can be found in Chapter 2.