Fourier transforms are used by a large number of customers at the Danish Computer Centre for Research and Education (UNI.C) in Lyngby, Denmark. Two excellent FFT Libraries are offered. The NAG Library [11, which is a general purpose numerical library, has powerful FFT subroutines in the chapter C06. The Siemens VP FFT Library [2, 3] has recently been installed for the UNI.C AMDAHL VP 1200 computer. The benchmarks clearly show a significant performance increase (in some cases by a factor of 3) with the new NAG Mark 15 Library as compared to the older NAG Mark 13, although this is offset by some loss of performance in other cases. The Siemens Library is shown to perform fastest for almost all cases studied.