The interactions of charged polymers, polyelectrolytes, on surfaces and in solution have gained a lot of attention throughout the years. Depositing alternating layers of oppositely charged polyelectrolytes forms multilayers, PEMUs. Their association in solution leads to phase separations forming materials termed polyelectrolyte complexes, PECs. The repeat unit of the oppositely charged polyelectrolyte can be varied. The functional group linked to the repeat units dictates the strength of ion pairing. Their self-assembly in solution creates polymer nanoparticles and, in a network, produces polymer nanocomposites. These materials have been used for a variety of applications such as coatings, electronic devices, and drug delivery systems. Adding a certain salt concentration to the material can influence the polymer/polymer interaction within the complex. This work features three main interfaces at which polyelectrolyte interactions were studied: on a substrate through PEMUs, in solution through polymer nanoparticles and in nanocomposites. The pairing strength of a polythiouronium polyelectrolyte having a positively charged thiouronium group, like the guanidinium, was evaluated by the formation of PEMUs at various salt concentrations. Polyguanidiniums such as polyarginine are known to form like-charge ion pairings and hydrogen bonding. The same unusual characteristics cause the strong interactions of the polythiouronium with polyanions. High concentrations of salt dissociated the strong polythiouronium/polyanions complexes. The polythiouronium was selected to seek another example of a strong polycation and to form a PEMU with polyzwitterions, which are known for their weakly interacting strength and net-neutral charge. The second interface consists of the self-assembly of a polydimethylsiloxane (PDMS) with positively charged end groups in solution, and as a nanofiller, in a nanocomposite. Polydimethylsiloxane is biocompatible and highly hydrophobic; however, when self-assembled exhibits poor stability and tends to collapse. Introducing a glassy shell of polystyrene sulfonate enhances the stability of these polymer nanoparticles and improves the mechanical properties of the nanocomposite which exhibited thermal resistance. Using x-ray scattering and atomic force microscopy, the nanocomposite's unique morphology was deduced. Its hydrophobic properties were probed using a solvatochromic dye and were used to encapsulate inorganic nanoparticles. The gold nanoparticles are hydrophobically capped and they appeared to be encapsulated within the PDMS core. Their assembly was studied by x-ray scattering in the presence of a PSS shell in solution and the loading capability of the nanoparticle was determined by transmission electron microscopy (TEM). The newly formed material is a novel polymer nanoparticle/polymer nanocomposite that takes advantage of the PDMS malleability and biocompatibility. The nanoparticle can thus be used for biological applications such as bio-imaging and sensing. The last section compares the interactions of different polycations with lipid layers (monolayer or bilayer) to mimic the mechanism of surface sterilization. The functional group varied between a primary or tertiary ammonium, a guanidinium and two different thiouronium groups. Ammoniums are known for their ability to disrupt lipid bilayers and for their use in disinfectants. Polyarginine, a strong positively charged polymer, is recognized for its cell-penetrating ability due to its unique salt-bridge like interactions. Radioactive unilamellar phosphocholine vesicles were prepared and used to deposit a monolayer on a hydrophobic scintillator and a bilayer on a hydrophilic one. The interactions of the layers and the deposition were monitored by radiocounting. Polyarginine and the polymer with a higher composition of a hydrophobic thiouronium exhibited the strongest interactions. These investigations of the interactions between polyelectrolytes at different interfaces serve as a guide to design a variety of functional materials. Understanding the structure-property polymer relationship allows the build-up of materials with tuned properties.