The book presents the Triple-Decker Model that describes the interactive mechanism among key constituents of English for Specific Purposes Ability (ESPA): language knowledge, strategic competence and background knowledge (nursing knowledge). First, the model reveals that ESPA constituents can be assigned to three groups according to their roles in determining ESPA in reading: automators (language knowledge and background knowledge) that respond most directly, assistants (the cognitive aspect of strategic competence) that come to assist when automators are insufficient or boggle down, and regulators (the metacognitive aspect of strategic competence) that supervise all cognitive activities. Second, the model demonstrates the effect of strategic competence and background knowledge on ESPA fluctuated with the continuous increase of language knowledge. The book also demonstrates the use of two innovative analytical techniques: composite scores based on bifactor multidimensional item response theory for scoring ESP reading tests and the multi-layered-moderation analysis (MLMA) for detecting linear and nonlinear moderation relations.