A downhole communication system for communication between a first and second location in a subsea oil and/or gas well installation. The oil and/or gas well installation comprises out of hole metallic structure comprising a riser 3 running upwards away from the mudline ML, and downhole metallic structure 2 running down into the well. The communication system is arranged so that at least part of a signal path for communications between the first and second locations is provided by the downhole metallic structure 2 such that, in use, data to be communicated between the first and second locations is carried by electrical signals in the downhole metallic structure 2. The communication system further comprises a first noise cancellation arrangement 5 arranged for sensing a noise signal generated in the out of hole metallic structure and arranged for applying a corresponding noise cancelling signal to the out of hole metallic structure or the downhole metallic structure to inhibit introduction of electrical noise into the downhole metallic structure 2 from the riser 3.