The aim of this thesis is to shed light on the Oppel-Kundt, vertical-horizontal and Helmholtz’s squares illusions. Eleven psychophysics experiments and one final fMRI study were conducted in order to inform current literature regarding these illusions. Experiments 1-5 investigated the effect of the number, size and position of vertical lines on the Oppel-Kundt illusion. Whereas five vertical lines allow for veridical size judgment of a horizontal line, a relatively constant and significant Oppel-Kundt 5% effect is observed with eight to twelve vertical lines. Evidence from these experiments also demonstrates that when one vertical line crosses a horizontal, ‘bisection’ decreases the perceived size of the horizontal by 7-13%. However, no reduction in the perceived size of the horizontal line in an inverted ‘T’ configuration is observed, challenging the ‘bisection’ component of the vertical-horizontal illusion as described by Mamassian & de Montalembert (2010). Experiments 6-7 showed that the increase in the perceived size of a horizontal line in an Oppel-Kundt figure can only be partly explained in terms of repulsion between adjacent lines. Specifically, whereas the size of the Oppel-Kundt illusion is 5%, displacement of adjacent lines can only account for 0.5%. Experiments 8-11 assess Mamassian and de Montalembert’ s (2010) simple model of the vertical-horizontal illusion and propose the new ABC model consisting of three components; anisotropy, abutting and bisection which were found to affect the perceived size of lines by +7%, +9% and -7% respectively. Finally, an fMRI study was carried out to investigate whether activity in V1 can be linked to perceptual experience. Results generated revealed a significant effect in V1 associated with physical differences between visual stimuli rather than perceived differences. We concluded that intrinsic processing in V1 is not responsible for inducing illusion-related activity and it is likely that feedback from other areas dominates results of other studies. The findings of this thesis have important implications on theories of visual illusions and processing of illusory-related percepts in the brain.