Abstract: Somenon-treatment variables that affect the outcomeof a disease are often called covariates. These covariables should be consideredin the design andanalysis of clinical trials to obtain unbias conclusion.To ensure that any observed treatment effect is not influenced by an imbalances in baseline characteristics, both pread-justment and postadjustment are providedin the design stage andanalysis stage of the trials respectively.They can improve the credibility of the trial results andincrease the statistical efficiency.Based on a few papers published about adjustment for covariates and somedocumentations of the International Conference on Harmonization (ICH), wereview the concepts, methods and procedures for ad-justment of treatment effects for the influence of covari-ates.The statistical issues on the application of adjust-ment are especially discussedin great depth.

Key words: clinical trial, treatment effect, covari-ate, stratification, minimization, stratified tests, regres-sion model, ICH