© 2007 IOP Publishing Ltd. We investigate the behaviour of ellipsoidal nanoparticles adsorbed at liquid-liquid interfaces and under the influence of an external field. We propose a simple thermodynamic model that predicts the equilibrium orientation of the particles as a function of the field strength and the interfacial tensions. We show that the interplay of particle geometry, particle size and field strength results in discontinuous orientational transitions, which are characterized by a discontinuous change of the particle orientation at a specific field strength. We also introduce a Monte Carlo methodology to investigate the orientational behaviour of anisotropic nanoparticles at interfaces. The simulation results show that the theory very accurately predicts the behaviour of anisotropic nanoparticles at intermediate field strengths.