The objective of this pal)er is to [brmalize the intuition al)out l,he comph;xity of syntactic structures. We propose a definition of structm:al COml)h'xity such that sentences ranked by our definition as more COml)h;x are gen(;rally more diI'ficult lbr humans to process. We justify the definition by showing how it is ahle to account for several seemingly unrelated phenomena in natural languages.