This book provides a detailed examination of the phonetics and phonology of consonant strength, drawing data from parallel acoustic and articulatory studies of English and Spanish, as well as a cross-linguistic survey of lenition and fortition. Explicit predictions about strength are tested for consonants of all manners of articulation, revealing evidence for aspects of several prevalent phonological views of lenition. The acoustic study analyzes durations, intensity and spectral properties; the articulatory study measures tongue-to-palate contact using electropalatography. Phonetic patterns that characterize consonant strength are located. Some phonetic characteristics pattern simply as general indicators of manner of articulation, while others pattern clearly by position (initial or medial, pre-stress or non-pre-stress). The phonetic studies reveal both similarities to phonological weakening and differences. While an elaborated sonority hierarchy best describes phonological weakening, it is a scaled-down hierarchy that captures phonetic weakening. Phonologically, voicing and fricativization are common steps in lenition, but the phonetic studies find little evidence of either. Despite the robust phonetic effects of stress, it does not frequently condition historical change. Because of their different effects, stress is argued to be a phonetic environment while word position is argued to be phonological. This book will be of interest to scholars of phonology, phonetics, and the phonetics-phonology interface. It includes a detailed discussion of methodology, as well as illustrative spectrograms and palatograms.