The cyclic imine toxins are a heterogeneous group of marine natural products sharing common macrocyclic features and the presence of an imine moiety, which is presumed to be the bioactive pharmacophore. This toxin group is known to comprise gymnodimines [1], pinnatoxins [2], prorocentrolides [3,4], pteriatoxins [5], spirolides [6], and spiro-prorocentrimine [7]. Comparison of the skeletons of the six cyclic imine toxin subclasses shows a high degree of structural similarity (e.g. ∼70% homology between pinnatoxins and spirolides) (Figure 26.1).