The Stratosphere-Troposphere Exchange (STE) of air mass and chemical species is an important process influencing the effects of trace chemical species emitted from natural and anthropogenic sources on the atmospheric composition.  STE may affect climate by altering the vertical distributions of greenhouse gases in the upper troposphere and low stratosphere.  Dynamical, chemical and radiative coupling between stratosphere and troposphere are among the many important processes that must be understood for prediction of global change. The paper reviews some main progresses in STE studies focusing on scale questions, researches in tropic and mid-latitude areas and also in China.  STE are multi-scale interaction processes. The wave in the extra-tropical stratosphere induced forces drive a kind of global-scale circulation, which is a useful measure of global-scale stratosphere-troposphere exchange, especially on seasonal or longer timescales.  But it is not suitable for describing short timescale processes. The circulation in the extra-tropical lower stratosphere cannot be simple regarded as zonal averaged. The distribution of some trace species cannot be simulated exactly if the model excludes the latitudinal asymmetry synoptic scale processes.  The theory of “leaky pipes” presents a new quantitative diagnostic framework to interpret measured results and compare them with model results. Tropopause folding and cut-off low in mid-latitude were well studied in the past.