In this chapter, the nature of age-related slowing in speeded motor performance is explored. In particular, experiments assessing movement parameterization are reviewed. In these studies, specific movement parameters (e.g., arm, direction, extent) comprising a motor program are assessed concerning their preparation, maintenance, restructuring and execution within a movement plan. An advantage of movement parameterization studies is that they assess cognitive processing latency to assess a movement response (reaction time, RT) distinct from the latency to complete the movement response (movement time, MT). In general, most speeded tasks assess both of these latencies in aggregate (and refer to this aggregate latency as simply "RT"). As such, parameterization studies allow a test of prevailing response slowing theories of aging using components of task performance. Separate "Brinley plot" regressions of RT and Total Time (TT, TT=RT+MT) from these studies reveals additive slowing, but nominal (if any) multiplicative slowing. Moreover, the intercept difference between the best-fitting RT and TT lines validates the additive impact of MT in these studies. Even at a global level, these studies are inconsistent with claims of negligible additive slowing (i.e., small positive or negative intercept), but substantial multiplicative slowing (slope approximating 2.0) for "nonlexical" tasks espoused by General Slowing theorists (e.g., Lima, Myerson, & Hale, 1991). In addition, review of the individual studies indicates what the Brinley plot approach misses: Age Group x Condition interactions from some of these studies actually indicate speed increases in elderly relative to young subjects, due to apparent differences in parameter preparation maintenance and restructuring processes for the two age groups.