In this conceptual contribution, thin functional coatings consisting of either pure polyelectrolytes (PELs) or complexes between oppositely charged PELs at model and applied substrates are outlined. Latter PEL/PEL complexes were deposited by two concepts. In a first well-known concept, PEL multilayers (PEM) were consecutively deposited according to the layer-by-layer (LbL) technique. In a second less known concept, PEL complex (PEC) nanoparticles (NPs) preformed by mixing polycation (PC) and polyanion (PA) solutions were deposited in one step. Both concepts based on binary oppositely charged PELs are compared to one based on a single polycation system. Examples shall be given on adhesiveness, nanostructure, and biomedical applications of PEM and PEC NP coatings. In situ attenuated total reflection (ATR) infrared (IR) spectroscopy, circular dichroism (CD), and scanning force microscopy (SFM) were used for molecular, optical, and microscopic characterization. At first, results on the adsorbed amount and wet adhesiveness of pure (single-component) PEL coatings as a function of charge density are given to motivate coatings of mixed oppositely charged PELs. Second, the wet adhesiveness of PEM and PEC NP coatings of identical PEL compounds in aqueous media varying the molar charge ratio (n-/n+) and the deposition step z, respectively, is compared. Upon comparing the three PEL deposition concepts, it is suggested that the lack or absence of excess charge at the PEL/surface interface is one of the main factors for the wet adhesiveness of all pure PEL, PEM, and PEC NP coatings. Finally, the potential of PEM and PEC NP coatings for biomedical applications is outlined. Concerning biopassivation, PEM coatings excessed or terminated by PA repel proteins with low isoelectric points. Concerning bioactivation, PEM coatings loaded with antibiotics as well as PEC NP coatings loaded with therapeutic bisphosphonates showed retarded, optionally temperature responsive drug release for applications in acute surgery and bone healing, and immunoglobulin/PEL complex coatings might open theranostic applications.