The advantages and limitations of two versions of the pulse technique for laboratory measurements of low permeabilities (10-6 to 10-24 m2) are discussed. The transient pulse technique developed by Brace et al. [1968> is the preferred method because of the ease of data reduction. The modified transient pulse technique developed by Trimmer et al [1980> can be used when the assumptions inherent in the transient pulse technique are invalid. The systematic error associated with the transient pulse technique is a function of the ratio of the effective sample pore volume to the reservior volumes. If this ratio is less than 0.25, then the error associated with the transient technique is less than 10%. The modified transient pulse technique does not suffer from this systematic error. Criteria are also presented for optimizing the experimental configuration for measurement durations as well as ease of data reduction.