Reliable and physically consistent process models are necessary to design and optimize separation equipment. Such models are expected to deliver sufficient information on process characteristics, fill the gaps in understanding the underlying phenomena, and help avoid process malfunctions. Recent progress in computer technology has significantly increased modeling possibilities, especially concerning rigorous methods. This has resulted in high expectations for an accurate and consistent description of separation columns. However, rigorous modeling of separation equipment remains difficult. This is why the spectrum of currently applied modeling methods is large and encompasses both simplified and sophisticated concepts. The choice of an appropriate model depends on different criteria, e.g., possibility of solving the governing mathematical model, availability of process data and model parameters, proper software and computer facilities, user experience, and expectations concerning accuracy and predictivity. It is essential to clearly understand the potential and limitations of different process models. In this contribution, established and widespread modeling methods applied in the fluid separation area are reviewed and critically evaluated. In addition, the complementary modeling concept is presented.