|Abstract:||This paper studies the processes of spirantization and devoicing in Spanish. It argues that the aim of these processes is twofold. Phonologically, they serve to simplify marked linguistic structures, and phonetically, they aid to reduce muscular effort in the articulation of segments intended to have a complete oral closure with simultaneous vocal fold vibration. It is assumed that the phonetic component is part of the grammar and that certain sound patterns are determined at this low level by principles specific to this component that operate on surface phonological representations. Markedness constraints are phonological principles that are phonetically grounded, whereas effort-reduction constraints pertain solely to the phonetic component. At both levels, voiced stops are problematic because they contain a complete oral closure, which is the greatest impediment to voicing. Spanish chooses to either give up their voicing, or keep it but loosen the complete oral closure. Phonologically, both stricture and voicing are lost in non-prominent positions but maintained in prominent ones, so that voiced stops may retain their distinctiveness. Phonetically, the stricture of voiced stops must be loosen in all contexts that do not facilitate articulatory ease, so that the crucial effort threshold enforced by the constraint ranking of the phonetic component may be respected. By allowing co-articulation, speech rate and register to play a role in the sound component of the grammar, this analysis accounts for the gradiency, and great deal of cross dialectal and dialect internal variation exhibited by Spanish spirantization.