Mechanical ventilation, like many other aspects of medicine, remains more of an art than a science. Part of the reason for this is because technological innovation in ventilator design has outpaced clinical practice and the acquisition of evidence for appropriate application of new capabilities. Nevertheless, the clinician's responsibility is to understand the fundamental concepts of biology and physics that underpin mechanical ventilation, the new capabilities of ventilators, and how best to support patient needs and optimize outcomes. What distinguishes the technical capabilities of ventilators is primarily the range of ?modes of ventilation? they offer. There are nearly 300 names of modes representing about 50 unique mode types. This level of complexity can be managed best by application of a formal classification system or taxonomy. This chapter describes the history of ventilator classification systems, research data supporting the most current approach, an abbreviated vocabulary, and a description of 10 fundamental concepts of biology and physics necessary to understand mechanical ventilation. These concepts build on one another to yield a complete taxonomy of modes. This taxonomy will be used to classify all the available modes on several of the most popular ventilators used around the world, providing much more information than is usually available from the operators' manuals. Importantly, this information allows recognition of identical modes that have different names on different ventilators. Having established a means of classification, we then provide explanations of the specific technological capabilities of currently available modes that may be used to simplify the task of comparing and contrasting the operational features of ventilators, and ultimately, to select the most appropriate mode for a given clinical situation.