A broad range of gel types and classifications exist with traditional applications like in jam, sweets and dairy products as well as emerging applications, for example, as matrix for encapsulation of bioactives. Molecular interactions are the basis for gel formation. They vary depending on the type of the gelling agent and result in very different microstructures.Diffusion plays a crucial role within the biopolymer gels and, because they are open systems, for the mass exchange between the gel and its environment. A broad variety of methods is required for the investigation of the complex mechanisms and a clear definition of the main terms is necessary for understanding and scientific exchange in this field.