Abstract:We introduced briefly the methodological key points of cohort study,and focused on the evidence level,methodological pros and cons as well as challenges of cohorts for clinical therapeutic effect evaluation.We discussed the special methodological issues related to clinical effectiveness evaluation of traditional Chinese medicine(TCM),including classification of TCM exposure levels,selection of outcomes,initial exposure and thorough exposure,control of migration of cohorts,use of blinding and placebo,follow-ups and some statistics issues.