In this thesis we study prefetching and buffer management algorithms for parallel I/O systems. Two models of lookahead, global and local, which give limited information regarding future accesses are introduced. Two configurations of the I/O buffer, shared and distributed, are considered, based upon the accessibility of the I/O buffer. The performance of prefetching algorithms using the two forms of lookahead is analyzed in the framework of competitive analysis, for read-once access patterns. Two algorithms, PHASE and GREED, which match the lower bounds are presented. A randomized version of GREED that performs the minimal expected number of I/Os is designed and applied to the problems of external sorting and video retrieval. Finally the problem of designing prefetching and buffer management algorithms for read-many reference strings is examined. An algorithm which uses randomized write-back to attain good expected I/O performance is presented.