A system for forming a fistula comprising: a first catheter configured to be advanced through a first vessel, the first catheter comprising one or more first catheter magnetic elements and a fistula-forming element; a second catheter configured to be advanced through a second vessel, the second catheter comprising one or more second catheter magnetic elements, wherein the one or more first catheter magnetic elements and the one or more second catheter magnetic elements are configured to draw the first catheter and the second catheter together; characterised in that the system further comprises one or more magnetic field sensors configured to output a magnetic field signal indicative of a magnitude of a magnetic field produced between the one or more first catheter magnetic elements and the one or more second catheter magnetic elements; a user output device configured to output a ready signal; and a controller communicatively coupled to the one or more magnetic field sensors and the user output device, wherein the controller is configured to: receive the magnetic field signal from the one or more magnetic field sensors; determine a distance of separation of the first catheter from the second catheter based on the magnetic field signal from the one or more magnetic field sensors; and output the ready signal with the user output device in response to determining that the distance of separation of the first catheter from the second catheter is a predetermined distance or less, wherein the ready signal is indicative of proper alignment and position of the first catheter and the second catheter to form the fistula with the fistula-forming element. a first catheter configured to be advanced through a first vessel, the first catheter comprising one or more first catheter magnetic elements and a fistula-forming element; a second catheter configured to be advanced through a second vessel, the second catheter comprising one or more second catheter magnetic elements, wherein the one or more first catheter magnetic elements and the one or more second catheter magnetic elements are configured to draw the first catheter and the second catheter together; characterised in that the system further comprises one or more magnetic field sensors configured to output a magnetic field signal indicative of a magnitude of a magnetic field produced between the one or more first catheter magnetic elements and the one or more second catheter magnetic elements; a user output device configured to output a ready signal; and a controller communicatively coupled to the one or more magnetic field sensors and the user output device, wherein the controller is configured to: receive the magnetic field signal from the one or more magnetic field sensors; determine a distance of separation of the first catheter from the second catheter based on the magnetic field signal from the one or more magnetic field sensors; and output the ready signal with the user output device in response to determining that the distance of separation of the first catheter from the second catheter is a predetermined distance or less, wherein the ready signal is indicative of proper alignment and position of the first catheter and the second catheter to form the fistula with the fistula-forming element. receive the magnetic field signal from the one or more magnetic field sensors; determine a distance of separation of the first catheter from the second catheter based on the magnetic field signal from the one or more magnetic field sensors; and output the ready signal with the user output device in response to determining that the distance of separation of the first catheter from the second catheter is a predetermined distance or less, wherein the ready signal is indicative of proper alignment and position of the first catheter and the second catheter to form the fistula with the fistula-forming element.