Despite the pervasiveness of lenition in the sound systems of natural language, this class of patterns has eluded adequate characterization in previous theories of phonology. Specifically, previous theories have failed to capture formally the phonetic unity of the various lenition processes (e.g. degemination, voicing, spirantization, debuccalization, deletion), or to account for the environments in which lenition typically occurs that presents a unified approach to consonant lenition, wherein particular lenition patterns arise from Optimality Theoretic conflict between a principle of effort minimization and faithfulness to auditory features, in combination with (perceptually-based) fortition constraints. It is further demonstrated that this effort-based approach straightforwardly accounts for a number of generalizations, drawn from a survey of 272 grammars: Geminate stops never lenite unless they concomitantly degeminate. Unaffricated stops never synchronically spirantize to strident fricatives. All else being equal, lenition occurs more readily the greater the openness of the flanking segments (the widely attested pattern of intervocalic lenition being a special case). Lenition occurs more readily the faster or more casual the speech. The approach is illustrated with case studies of lenition in Tumpisa Shoshone and Florentine Italian. This book represents a significant contribution to the current debate on the role of phonetic optimalization in phonological theory.