Chemical reactions initiated in the direct friction contact zone relate to the tribochemistry which deals with the chemical changes of both solid mating elements and lubricant molecules due to the influence of boundary and/or mixed lubrication operating conditions. Tribochemical reactions differ significantly from those initiated by temperature only. The major thrust of this paper is to review and discuss a wide variety of existing data and some new experimental results aiming at answering several important questions. How the present knowledge of tribochemistry can be applied in elucidation of detailed mechanisms by which boundary lubricant compounds reduce wear? How the boundary friction process can affect base oils, particularly model base oils, for example, n-hexadecane in comparison with tribochemistry of carboxylic acids and some alcohols. What is the part of the Negative-Ion-Radical Action Mechanism (NIRAM) in initiating tribochemical reactions and what are their specific intermediates? The present paper also considers major issues of triboemission and related phenomena/processes.