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Global   products   of   vegetation   green   Leaf   Area Index  (LAI)  and  Fraction  of  Photosynthetically  Active  Radiation  absorbed  by  vegetation  (FPAR)  are  being  operationally produced  from  Terra  and  Aqua  Moderate  Resolution  Imaging Spectroradiometers  (MODIS)  at  1-km  resolution  and  eight-day frequency.   This   paper  summarizes  the   experience  of   several collaborating  investigators  on  validation  of  MODIS  LAI  products  and  demonstrates  the  close  connection   between  product validation and algorithm refinement activities. The validation of moderate  resolution  LAI  products  includes  three  steps:  1)  field sampling representative of LAI spatial distribution and dynamic range within each major land cover type  at the validation site; 2) development  of  a  transfer  function  between  field  LAI  measurements  and  high  resolution  satellite  data  to  generate  a  reference LAI map over an extended area;  and 3) comparison of MODIS LAI  with  aggregated   reference  LAI  map  at  patch  (multipixel) scale  in  view  of  geo-location  and  pixel  shift  uncertainties.  The MODIS  LAI  validation  experiences,  summarized  here,  suggest three  key  factors  that  influence  the  accuracy  of  LAI  retrievals: 1) uncertainties in input land cover data, 2) uncertainties in input surface  reflectances,  and  3)  uncertainties  from  the  model  used to  build  the  look-up  tables  accompanying  the  algorithm.  This strategy  of  validation  efforts  guiding  algorithm   refinements  has led to progressively more accurate LAI products from the MODIS sensors aboard NASA’s Terra and Aqua platforms.