Impedance flow cytometry apparatus comprising: a flow channel for carrying a flow of fluid comprising particles suspended in an electrolyte from an inlet to an outlet; a first electrode group and a second electrode group, each electrode group providing first and second current paths through fluid flowing in the flow channel; the apparatus characterised in that: each electrode group comprises: a first signal electrode (60a, 60b) to provide to the first current path a first electric signal of frequency, magnitude and phase; a second signal electrode (64a, 64b) to provide to the second current path a second electrical signal of substantially equal frequency and magnitude as the first electrical signal and of opposite phase to the first electrical signal; and one or more measurement electrodes (62a, 62b, 66a, 66b) to detect current flow in the first current path and the second current path and produce a summed signal representing the sum of the current flow in the first current path and the current flow in the second current path; wherein the first electrode group produces a first summed signal and the second electrode group produces a second summed signal; and that the apparatus further comprises: circuitry to determine a differential signal representing the difference between the first summed signal and the second summed signal; in which the signal electrode and the measurement electrode of at least one current path are located on a same side of the flow channel to provide the current path substantially along a direction of flow in the flow channel from the inlet to the outlet. a flow channel for carrying a flow of fluid comprising particles suspended in an electrolyte from an inlet to an outlet; a first electrode group and a second electrode group, each electrode group providing first and second current paths through fluid flowing in the flow channel; the apparatus characterised in that: each electrode group comprises: a first signal electrode (60a, 60b) to provide to the first current path a first electric signal of frequency, magnitude and phase; a second signal electrode (64a, 64b) to provide to the second current path a second electrical signal of substantially equal frequency and magnitude as the first electrical signal and of opposite phase to the first electrical signal; and one or more measurement electrodes (62a, 62b, 66a, 66b) to detect current flow in the first current path and the second current path and produce a summed signal representing the sum of the current flow in the first current path and the current flow in the second current path; wherein the first electrode group produces a first summed signal and the second electrode group produces a second summed signal; each electrode group comprises: a first signal electrode (60a, 60b) to provide to the first current path a first electric signal of frequency, magnitude and phase; a second signal electrode (64a, 64b) to provide to the second current path a second electrical signal of substantially equal frequency and magnitude as the first electrical signal and of opposite phase to the first electrical signal; and one or more measurement electrodes (62a, 62b, 66a, 66b) to detect current flow in the first current path and the second current path and produce a summed signal representing the sum of the current flow in the first current path and the current flow in the second current path; wherein the first electrode group produces a first summed signal and the second electrode group produces a second summed signal; a first signal electrode (60a, 60b) to provide to the first current path a first electric signal of frequency, magnitude and phase; a second signal electrode (64a, 64b) to provide to the second current path a second electrical signal of substantially equal frequency and magnitude as the first electrical signal and of opposite phase to the first electrical signal; and one or more measurement electrodes (62a, 62b, 66a, 66b) to detect current flow in the first current path and the second current path and produce a summed signal representing the sum of the current flow in the first current path and the current flow in the second current path; wherein the first electrode group produces a first summed signal and the second electrode group produces a second summed signal; and that the apparatus further comprises: circuitry to determine a differential signal representing the difference between the first summed signal and the second summed signal; in which the signal electrode and the measurement electrode of at least one current path are located on a same side of the flow channel to provide the current path substantially along a direction of flow in the flow channel from the inlet to the outlet. circuitry to determine a differential signal representing the difference between the first summed signal and the second summed signal; in which the signal electrode and the measurement electrode of at least one current path are located on a same side of the flow channel to provide the current path substantially along a direction of flow in the flow channel from the inlet to the outlet.