Two topics on the tribology issues of solid materials concerning trace water are introduced and discussed in this article. Firstly, the influences of the trace water, which adsorbs on solid surfaces due to humidity in atmospheric gas, on the tribological phenomena are reviewed. The adsorbed water is introduced as a major influencing factor on the tribological phenomena especially in terms of physical effects and the importance of its quantitative evaluation is explained. Secondly, the influences of trace water in gaseous environment on the tribological phenomena of solid materials are reviewed. Some important experimental techniques to control the trace water in an experimental gaseous environment for the sliding tests are suggested
