Methods and circuitry for implementing high speed first-in first-out (FIFO) structures. In one embodiment, a FIFO is disclosed that allows the frequency of one clock, e.g., the write clock, to be different than (e.g., half) that of the other (read) clock. In another embodiment a FIFO is presented that can be set and/or reset asynchronously. Other embodiments are disclosed wherein the read and write pointers are effectively monitored to ensure proper timing relationship, to detect loss of clock as well as to detect other abnormal FIFO conditions.