A clock synchroniser, and clock and data recovery apparatus incorporating the clock synchroniser, are described, together with corresponding clock synchronisation methods. The clock synchroniser incorporates an elastic buffer. A received clock signal is used to clock data into the buffer, and a locally generated clock is used to clock data out of the buffer. The local clock is synthesised using a PLL, and a fill-level signal from the elastic buffer is used to control to local clock frequency to maintain a desired average quantity of data in the buffer, thereby achieving synchronisation of the received and local clocks. In preferred embodiments the fill-level signal is used to control a variable divider in the feedback path of the PLL, which is supplied with a highly stable reference signal. A synchronised, and low-jitter local clock is thus produced. Preferably, the elastic buffer employs counters of relatively wide word width, and a storage array of much reduced depth, read and write pointers being provided by just a few of the least significant bits of the words.