Leaf Area Index（LAI） is a key parameter for terrestrial remote sensing. Indirect methods are fast and efficient

and they arewidely used in ground measurement and the remote sensing validation of the LAI. These methods servea large number of industries and fields

as theircitation count is far greater than theirliterature count. This paper comprehensively reviews the development and current status of indirect LAI measurement with regard to theories

algorithms

instruments

and influence factors. The latest progress and future prospects are summarized and discussed onthe basis off our challenges

including clumping/scale effect

leaf angle distribution

woody component

and slope effect. First

the basic concepts and theoretical background are summarized to provide an overview of indirect LAI measurement. The basic concepts include LAI definition

gap probability

leaf angle distribution

leaf projection function

clumping index

needle-to-shoot area ratio

and woody-to-total area ratio. Theoretical derivations begin with the introduction of Beer’s law on indirect LAI measurement

followed by three essential adaptations concerning leaf angle distribution

clumping effect

and woody component. Second

representative methods for quantifying the clumping effect are reviewed

with the clumping effect beingthe most important factor. These methods consist of the finite-length averaging method

the gap-size distribution method

the combination of finite-length averaging and gap-size distribution method

and the path length distribution method. Third

representative instruments for indirect measurement are reviewed. These instruments include LAI-2000

TRAC

line quantum sensors（Accu PAR

Sun Scan

and LAInet）

DEMON

imaging instruments（Hemi View

MVI

and MCI）

and Li DAR. Airborne and spaceborne Li DAR are also reviewed because they also use Beer’s law as the indirect LAI measurement. Finally

four key factors that limit indirect LAI measurement accuracy are discussed. These factors are the clumping/scale effect

leaf angle distribution

woody component

and slope effect. The mixed effects of these factors are also explained.The clumping effect has attracted the most attention in the community

with several related methods having beendeveloped and being widely used in field measurement.The non-randomness inside canopies and the scale effect of measurement is also worth studying further. The theory of leaf angle distribution is well-developed

whereas the spherical distribution assumption（G≡0.5） is often used because of the inconvenience of measurement.The spherical distribution assumption should be used with caution because it is only valid near a zenith angle of 57.3°. The efforts to develop fast and automatic methods for measuring leaf angle distribution show greatpromise. The importance of woody components is widely recognized

but they are always ignored in measurement because commercial instruments are incapable of distinguishing woody components.Near-infrared technology and instruments should be applied to ground measurement to distinguish the LAI from the plant area index. The slope effect has attracted more attention than before. Research shows that it canbe ignored when the slope is less than 30°; otherwise

it could be a moderate source of error. Generally

making a break through in theories is difficult

although the main constraints have been identified and much progress hasbeen made. With a focuson these aspects

developing new instruments and calculation and validation methods

further exploring existing data

and finally improving the accuracy and stability of indirect LAI measurement are endeavors worth pursuing.