Leafareaindex(LAI)isakeyparameterofvegetationstructureinthefieldsofagriculture,forestry,andecology. OpticalindirectmethodsbasedontheBeer-Lambertlawarewidelyadoptedinnumerousfieldsgiventheirhigh efficiencyandfeasibilityforLAIestimation.Thesemethodshaveundergoneconsiderableprogressinthepast decades,therebymakingthemoperationalinground-basedLAImeasurementandeveninairborneestimation. However, several challenges remain, given the requirement of increasing accuracy and new applications. Clumpingeffectcorrectionattainedsignificantprogressforcontinuouscanopieswithnon-randomlydisturbed leaveswhilenon-continuouscanopiesarerarelystudied.Convenientandoperationalmeasurementofleafangle distributionandwoodycomponentsislacked.Accurateandcomprehensivevalidationsarestillverydifficultdue tothelimitationsofdirectmeasurement.Theintroductionofactivelaserscanningtechnologyisadrivingforce foraddressingseveralchallenges,butitsthree-dimensionalinformationhasnotbeenfullyexploredandutilized. In order to update the general knowledge and identify the possible error source, this study comprehensively reviewsthetemporaldevelopment,theoreticalframework,andissuesofindirectLAImeasurement,followedby currentmethods,instruments,andplatforms.Latestmethodsandinstrumentsareintroducedandcomparedto traditional ones. Current challenges, recent advances, and future perspectives are discussed to provide re- commendationsforfurtherresearch.