The Hall effect has been widely utilized to measure magnetic fields. The relatively simple geometry of a Hall element suggested the use of such a device on a microscale as a probe to examine magnetic fields of small structures. Hall probes are described which were constructed with a sensitive area about 10×10 &mgr;. Fields of less than 0.01 gauss were found to be measurable using evaporated bismuth films. Probe fabrication techniques are described, and the factors affecting probe performance are discussed. A number of instrumentation systems, which have proven suitable for measuring small dc magnetic fields and high frequency ac fields, are presented. High accuracy (1%) of field measurement is readily attained.