The past few years have seen an increasing focus on inhibitory processes in cognition (see Dempster, 1992; Dempster & Brainerd, 1995), especially in their impairment in certain patient populations (e.g., Beech, Powell, McWilliam, & Claridge, 1989; Cohen & Servan-Schreiber, 1992), in their development during childhood (e.g., Harnishfeger, 1995), and in their decline with normal aging (e.g., McDowd, Oseas-Kreger, & Filion, 1995). In this chapter, we briefly describe the inhibition deficit hypothesis of cognitive aging and discuss some logical and methodological issues that have complicated its investigation. We then present three aging studies across the different domains of short-term memory (Maylor & Henson, 2000), visual search (Watson & Maylor, 2002), and motor control (Schlaghecken & Maylor, 2005). In each case, it is argued that inhibitory processes are responsible for the effects of interest, namely, the Ranschburg effect (Crowder, 1968; Jahnke, 1969), the preview benefit in visual search (known as visual marking; Watson & Humphreys, 1997), and the negative compatibility effect (Eimer & Schlaghecken, 1998), respectively. What these three effects have in common is that, at some level, they all occur as a result of inhibitory processes that suppress responses to stimuli that are no longer relevant to current goals. The results show mixed support for the inhibition deficit hypothesis of aging, the current status of which is finally discussed in the light of these and other data.