A study by Ihssen, Linden and Shapiro (2010) increased visual short-term memory (VSTM) performance by either repeating an 8-item array, or presenting it across two 4-item sequential displays. These increases were suggested to occur due to decreases in overall inter-stimulus competition, enabled in a top-down and bottom-up manner respectively (Ihssen, Linden, Miller & Shapiro, 2015). These studies add to growing evidence that reducing competition in visual cortex may improve VSTM performance (see Shapiro & Miller, 2011). This thesis sought to further investigate the effects of competition on VSTM, through manipulation of both bottom-up and top-down elements of inter-stimulus competition. Bottom-up competition was manipulated by varying properties of to-be-remembered sequential stimulus displays; more specifically by presenting either a similar number of items in each display or several more items in one display than the other. Reducing overall inter-stimulus competition using this manipulation elicited increased VSTM performance. Further experiments considered the ability of participants to use top-down cues to manipulate VSTM contents. The novel cueing paradigm used in Chapter 5 revealed specific impairments in inhibiting the encoding of new stimuli at short notice, whilst holding items in VSTM. In contrast, VSTM could be easily updated when participants were required to remove old items and encode new ones in their place. The effect of ageing on the top-down and bottom-up aspects of Ihssen et al.’s findings was also investigated, to determine whether the multiple stimulus display presentation may help to compensate for VSTM decreases seen in older adults (e.g., Jost, Bryck, Vogel & Mayr, 2011). In addition, the ERP experiments reported in Chapter 3 manipulated low-level competition by varying inter-stimulus proximity. Significantly, evidence of competition was found in the initial feedforward V1 response, using a novel method developed to assess competition throughout visual cortex by measuring early visual ERP components. This method has future potential for assessing competition present in visual cortex, and the contribution of feedforward and feedback processes at different time points to perceptual and cognitive processes.