Tooth wear is a multifactorial condition, leading to the loss of dental hard tissue, ie, enamel and dentin. Tooth wear can be divided into the subtypes mechanical wear (attrition and abrasion), and chemical wear (erosion). Due to its multifactorial etiology, tooth wear can manifest in many different ways. The condition can therefore be difficult to diagnose and manage. A systematic approach is a sine qua non. In the Tooth Wear Evaluation System (TWES) described in this article (the first in a series of two), all the necessary tools for a clinical guideline are presented in various modules. This allows the clinician to perform a state-of-the-art diagnosis, in both a general practitioner and a referral setting. To avoid the risk of cumbersome usage, the clinician can select only those modules appropriate in a given setting. The modules of the TWES match each other, which is indispensable and essential when different modules are being compared. With the TWES, it is possible to recognize the problem (qualification), grade its severity (quantification), diagnose the likely causes, and monitor (the progress of) the condition. In addition, a proposal for the classification of tooth wear is made. Further, it is possible to determine when to start a treatment, decide which kind of treatment to apply, and estimate the level of difficulty of a restorative treatment. In this first article, the diagnostic modules and proposed classification system are described. Keywords: clinical guideline, diagnosis, qualifying, quantifying, classification, tooth wear