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Ferromagnetic liquids are here defined as colloidal suspensions of single domain particles with diameters between 40-200 Å. The magnetic properties are best described as superparamagnetic and the classical theory of Langevin paramagnetism, modified to include a particle size distribution, applies to these systems. Liquids have been prepared with saturation magnetisations between 50 G and 1000 G, their viscosity increasing from approximately 1 cp to 104cp as the magnetisation increases. The colloidal particles have magnetic, hydrodynamic and physical sizes which are different. The mode of magnetisation may, therefore, be complex arising either through physical rotation of the particle (Brownian rotation) or a rotation of the magnetisation within an essentially static particle (Néel rotation). The actual process can have a profound effect on the properties of the ferromagnetic liquids. Ferromagnetic liquids have a viscosity which is anisotropic in a magnetic field and a magnetic birefringence which produces interesting domain observations in polarised white light. The use of ferromagnetic liquids in devices is very closely related to their stability in magnetic field gradients and a criterion in terms of the change in particle concentration in the field gradient ∇/n <1cm-1can be defined. Although stable ferromagnetic liquids are currently used in rotating shaft seals and as damping fluids the use of the unique properties of ferromagnetic liquids in devices has not yet been fully exploited.