In gas-liquid contact devices like absorbers, scrubbers, quench coolers or condensers, aerosols can be formed by spontaneous phase transitions, initiated by homogeneous or heterogeneous nucleation, if special process operation conditions lead to a metastable, i.e. a supersaturated state in the gas phase. Aerosol formation can impact severely the mass separation efficiency of gas-liquid contactors. This is demonstrated by experiments performed in semi-technical plants. The paper is aimed to identify strategies for understanding and describing the complex aerosol behaviour in gas-liquid contact devices. Operation conditions are identified under which supersaturation can arise, and the fundamentals of modelling aerosol formation and growth in gas-liquid contactors are discussed. The SENECA code developed by the authors allows to simulate aerosol formation and behaviour in contact devices as well as in multistage gas cleaning processes. Experimental results show that most of all important features of aerosol behaviour in flue gas cleaning and in condensation processes can be predicted with good accuracy by SENECA.