We have investigated the reliability of a variety of metal coated and semiconductor tips for use in conducting atomic force microscopy experiments in an ultrahigh vacuum (UHV) environment. In order to obtain reliable conduction data we find it necessary to first clean the tips using a short argon ion sputter. Scanning transmission electron microscopy is used to image tips after the conductivity experiments and found to be very useful for assessing tip wear and interpreting conductivity data. Tip reliability is found to be strongly dependent on the sample and the experimental conditions. Wear and contamination of the tip are found to be severe problems which are related to the tip-sample adhesion. We illustrate these effects and highlight some of the common reliability problems which we encountered using specific examples. In general, we find that metal coated tips are not reliable enough to obtain repeatable data, especially if lateral forces are exerted on the tip. Homogeneous semiconductor tips, once cleaned, are found to be satisfactory and a particular contrast with experiments performed in air is that Si tips can be used reliably. In addition we find that in UHV, conduction experiments may be reliably performed even at very low applied force, of order nano-Newtons. This is a clear advantage in comparison to experiments performed in air where surface contamination is present and applied forces on the order of micro-Newtons are often required to establish stable electrical contact. ©1998 American Institute of Physics.