The paper gives a survey on recent experimental and theoretical investigations on the tokamak TEXTOR concerning the influence of impurities on the plasma behaviour. A broad spectrum of diagnostics allows the measurements of the spatial distributions of densities, temperatures and radiation of impurity and background particles. The plasma density critical for development of the radiative instability observed in the core of ohmically heated plasmas in discharges with high-Z limiters is estimated on the basis of a simple analytical model. The results of modelling by the code RITM of plasmas polluted by low-Z impurities are presented. An interpretation of different stability of OH and additionally heated plasmas to detachment, of improvement of energy confinement with neon seeding is proposed. The importance of impurities in the plasma dynamics in the vicinity of strong sources of impurities through their influence on the electric field is discussed.