A wireless power transfer system comprising: one or more primary coils for generating an oscillating electromagnetic field for wirelessly transferring power to one or more secondary coils; and at least one of: (i) one or more emitting coils arranged to generate an electromagnetic field for exciting a nuclear quadrupole resonance in the biological material of an animal exposed to the electromagnetic field generated by the one or more emitting coils, and one or more detecting coils for detecting the absorption and/or emission of electromagnetic radiation by and/or from the excitation of the nuclear quadrupole resonance; or (ii) one or more emitting and detecting coils arranged to generate an electromagnetic field for exciting a nuclear quadrupole resonance in the biological material of an animal exposed to the electromagnetic field and to detect the emission and/or absorption of electromagnetic radiation from and/or by the nuclear quadrupole resonance; wherein the system is arranged, when the one or more detecting coils or the one or more emitting and detecting coils detect the absorption and/or emission of electromagnetic radiation by and/or from the excitation of the nuclear quadrupole resonance in the biological material of an animal, to prevent the one or more primary coils from generating, or to cause the one or more primary coils to reduce the amplitude of, the oscillating electromagnetic field. one or more primary coils for generating an oscillating electromagnetic field for wirelessly transferring power to one or more secondary coils; and at least one of: (i) one or more emitting coils arranged to generate an electromagnetic field for exciting a nuclear quadrupole resonance in the biological material of an animal exposed to the electromagnetic field generated by the one or more emitting coils, and one or more detecting coils for detecting the absorption and/or emission of electromagnetic radiation by and/or from the excitation of the nuclear quadrupole resonance; or (ii) one or more emitting and detecting coils arranged to generate an electromagnetic field for exciting a nuclear quadrupole resonance in the biological material of an animal exposed to the electromagnetic field and to detect the emission and/or absorption of electromagnetic radiation from and/or by the nuclear quadrupole resonance; (i) one or more emitting coils arranged to generate an electromagnetic field for exciting a nuclear quadrupole resonance in the biological material of an animal exposed to the electromagnetic field generated by the one or more emitting coils, and one or more detecting coils for detecting the absorption and/or emission of electromagnetic radiation by and/or from the excitation of the nuclear quadrupole resonance; or (ii) one or more emitting and detecting coils arranged to generate an electromagnetic field for exciting a nuclear quadrupole resonance in the biological material of an animal exposed to the electromagnetic field and to detect the emission and/or absorption of electromagnetic radiation from and/or by the nuclear quadrupole resonance; wherein the system is arranged, when the one or more detecting coils or the one or more emitting and detecting coils detect the absorption and/or emission of electromagnetic radiation by and/or from the excitation of the nuclear quadrupole resonance in the biological material of an animal, to prevent the one or more primary coils from generating, or to cause the one or more primary coils to reduce the amplitude of, the oscillating electromagnetic field.