Leaf area index (LAI) corresponds to one half the total green leaf area per unit horizontal ground surface area. It is one of the essential climate variables identified by the Global Climate Observing System (GCOS) and has been applied in a range of biosphere-atmosphere, biogeochemical, and ecological models as a primary biophysical and ecological parameter. This talk consists of three parts. The first part presents the LAI ground measurement methods. In the field, LAI can be estimated directly or indirectly: the direct methods measure LAI through litterfall or destructive sampling; the indirect methods estimate LAI via relationship with other more easily measurable parameters, such as canopy transmittance. The second part is about the LAI estimation from optical remote sensing technique. Current methods for estimating LAI are classified into three categories through: (1) the empirical relationship between LAI and vegetation indices; (2) inversion of canopy reflectance models; and (3) a hybrid inversion method. Following this, several major global LAI products and their spatial and temporal properties will be introduced. The last part of the presentation focuses on how to validate remote sensing LAI products. There are several validation schemes that develop uncertainty information about the global LAI products, such as the direct comparison method, the bridging method that makes comparison with upscaled high resolution products, and the cross-validation method.