High pT Hadron Correlation and No Correlation

An overview of the correlation among high-pT hadrons produced in heavy-ion collisions is presented. Emphasis is placed on the physical processes that can quantitatively account for the data on correlations in pT, η and ϕ on the near and far sides. Predictions are made on processes that have no observable correlations for hadrons produced at intermediate and higher pT at RHIC and LHC.