Amyloids are elongated supramolecular protein self-assemblies. Their formation is a non-covalent assembly process and as such is fully reversible. Amyloid formation is associated with several neurodegenerative diseases, and the reversibility is key to maintaining the healthy state. Reversibility is also key to the performance of fibril-based biomaterials and functional amyloids. The reversibility can be observed by a range of spectroscopic, calorimetric, or surface-based techniques using as a starting state either a supersaturated monomer solution or diluted fibrils. Amyloid formation has the characteristics of a phase transition, and we provide some basic formalism for the reversibility and the derivation of the solubility/critical concentration. We also discuss conditions under which the dissociation of amyloids may be so slow that the process can be viewed as practically irreversible, for example, because it is slow relative to the experimental time frame or because the system at hand contains a source for constant monomer addition.