An apparatus for heating a treatment region of a subject via the application of an alternating electric field, comprising: a first high voltage electrode and a second ground electrode each of which is connected to a generator operable to apply an alternating electric field between said electrodes; a cavity located between said first and second electrodes and configured to receive said treatment region; and one or more flowable materials that displace any air located between said first and second electrodes and said treatment region within said cavity; characterised in that said generator is operable to generate a signal that is substantially a sinusoid having a wavelength λ, wherein at least one point on said first electrode is located a distance of 1⁄4 λ or 1⁄4 λ plus a multiple of 1⁄2 λ from said generator so as to provide a substantially constant voltage between the first and second electrodes, wherein said substantially constant voltage comprises a voltage in which the difference between a voltage provided at a point on the first high voltage electrode compared to a voltage provided at each other point on the first high voltage electrode is less than ±10%, and wherein each of said flowable materials has a-dielectric constant and a dissipation factor that are calculated so as to obtain a substantially constant current passing between said first and second electrodes and through said treatment region so as to provide substantially even heating throughout the thickness of said treatment region, wherein said substantially constant current comprises a current in which a difference between a first current passing through said treatment region compared to each other current passing through said treatment region is less than ±25%. a first high voltage electrode and a second ground electrode each of which is connected to a generator operable to apply an alternating electric field between said electrodes; a cavity located between said first and second electrodes and configured to receive said treatment region; and one or more flowable materials that displace any air located between said first and second electrodes and said treatment region within said cavity; characterised in that said generator is operable to generate a signal that is substantially a sinusoid having a wavelength λ, wherein at least one point on said first electrode is located a distance of 1⁄4 λ or 1⁄4 λ plus a multiple of 1⁄2 λ from said generator so as to provide a substantially constant voltage between the first and second electrodes, wherein said substantially constant voltage comprises a voltage in which the difference between a voltage provided at a point on the first high voltage electrode compared to a voltage provided at each other point on the first high voltage electrode is less than ±10%, and wherein each of said flowable materials has a-dielectric constant and a dissipation factor that are calculated so as to obtain a substantially constant current passing between said first and second electrodes and through said treatment region so as to provide substantially even heating throughout the thickness of said treatment region, wherein said substantially constant current comprises a current in which a difference between a first current passing through said treatment region compared to each other current passing through said treatment region is less than ±25%.