This thesis research focuses on fundamental understanding regarding the morphological transitions of weak polyelectrolyte multilayers (PEMs) formed by the layer-by-layer (LbL) electrostatic assembly of oppositely charged polymers. he first part of this thesis focuses on patterning polyelectrolyte multilayers that are able to undergo transitions from continuous films to porous materials by using hydrogel stamps. The stamping process is able to locally etch and pattern the porous transition in the LbL films by using reactive wet stamping (r-WETS). It was found that r-WETS of PEMs can also enable the modification of chemical functionality. The second part is an investigation about morphological changes of weak polyelectrolyte multilayers assembled with PAH and PAA using r-WETS in which hydrogel stamp material was soaked into various salt solutions and then applied to the LbL films. Also, in this study we presented a novel strategy to create a continuous gradient structure in thickness or porosity along the lateral direction of the thin films using concentration gradient salt stamping. The third part is an investigation regarding the mechanism of the transition from a continuous morphology to a porous morphology within weak polyelectrolyte multilayers. These morphological changes were able to be created by both acidic and basic post-assembly treatments, showing various morphological transitions from the introduction of porosity to the collapse of these porous structures and the eventual dissolution of the films. A similar observation of morphological transitions in weak polyelectrolyte multilayers was obtained by applying an electric field to the films in the fourth part of this thesis. Exposure to an electric field resulted in the creation of a porous structure, which can be ascribed to local changes in pH and subsequent structural rearrangements of the weak polyelectrolyte constituents. The final part of this thesis is to make PEMs into nanostructured matrices for inorganic synthesis. Multilayers possessing ion-exchangeable carboxylic acid groups were used for binding metal catalysts such as platinum (Pt) nanoparticles (NPs) within the film. Therefore, polyelectrolyte multilayers were able to stabilize catalytic Pt NPs in order to increase the useful time of catalyst materials suitable for use in proton exchange membrane fuel cells.