In the evaluation of new oil fields there are, among others, three important characteristic parameters of the formation rocks. These parameters are: the porosity, the pore-size distribution and the permeability. NMR turns out to be the most appropriate technique to this end. Basic theory, measurements, their analyses and results are presented. It is interesting to compare the NMR results with those obtained by means of other techniques like the mercury injection core porosimetry (MICP) and the centrifuge. This last technique was recently developed in our laboratory and still is under test. Similarities and discrepancies are found, and tentative explanations are given. A better and more correct evaluation of the formation rocks is obtained combining the above mentioned analyses.