The needle penetrometer (NP) is a nondestructive, cheap and simple device which can be used indirectly to obtain the uniaxial compressive strength of extremely weak rocks. It requires little sample preparation and can be used in the field and laboratory and applied in natural and man-made structures where sampling is not allowed. The microstructural damage created by the needle, its shape and size, have been assessed using four rock types (marl, mudstone and tuff from Turkey and calcarenite from the Netherlands) and two types of needle. During needle penetration, very high compressive and shear stresses are developed in advance of the needle and normal to the shaft. In all the tested rocks, densification occurred in a zone some 0.4 and 1 mm ahead of the Maruto and Eijkelkamp needles respectively. The grains are crushed and compacted in a zone which appears like an extension of the needle tip (Eijkelkamp needle). As damage is local, the NP test is said to be non-destructive. The NP results in coarse grained rocks are far more variable than in fine grained rocks. The damage caused by a needle decreases with a decrease in the diameter of the needle and an increase in the slenderness of the needle.