Four experiments are reported in which the effect of complexity on short-term retention of visuo-spatial material was explored. The determinants of complexity can be separated into a quantitative factor, which sets an upper bound on complexity, and a structural factor, which reduces complexity. Variants of the Corsi blocks task were administered across the various experiments. Quantitative complexity was manipulated through the number of blocks on the board. Structural complexity was induced through the positioning of the blocks. Visuo-spatial span was found to be susceptible to both measures of complexity. Performance was inversely related to the number of blocks. Recall was also better when the blocks were positioned in a matrix than in a random fashion. Moreover, the effect of complexity was shown to be moderated by an interaction between structure and amount of information presented. These results demonstrate that complexity is an important characteristic of visuo-spatial working memory. This phenomenon may be used to further explore the properties of the visuo-spatial sketch pad, and advance its theoretical development.