Absorption of HI spectral lines against pulsars can be used to study many aspects of the structure of the absorbing clouds, the distribution of the different phases of the ISM along the line of sight and provide us with information on the structure of our Galaxy. I will discuss how these "easy in principal" but "difficult in practice" measurements are carried out. I will point out several pitfalls that have lead to previous claims of the detection of small scale structure in the ISM. GBT observations of the pulsar 0329+54 will be presented which have been used to create the first power spectrum of HI structure on AU scales. HI absorption measurements against the magnetar, XTE J1810-19, will be presented. These observations were used to determine the distance to this important source. They also show how the Galactic bar must be included in velocity-distance relationships in order to obtain a proper understanding of the structure of the ISM.