Polyelectrolyte (PE) brushes refer to surface tethered polyelectrolyte chains with sufficiently high grafting density such that the polymer chains are forced to extend from the surfaces. Most physicochemical properties of PE brushes are stimuliresponsive and they are thus able to create "smart" surfaces for various applications. In addition, many biointerfaces inside biological systems possess "brush-like" structures and may also carry many ionizable groups. PE brushes can serve as realistic models to better understand the behaviors of those biological interfaces. This dissertation describes the synthesis, characterization and applications for several types of PE brushes. Surface-initiated atom transfer radical polymerization in protic solvents was used to directly synthesize the PE brushes. A variety of characterization methods were used to understand the chemical structure and physical properties of the brushes. We were able to estimate the pKa of weak polyacid brushes and experimentally confirmed the variation of pKa inside the weak PE brushes. Poly(acrylic acid) (PAA) brushes were patterned on silica surfaces by a protocol modified from photolithography. The patterned PAA brushes were used to create arrays of biomolecules to study the signaling pathways of rat basophilic leukemia (RBL) mast cells. Cell adhesion on patterned PAA brush surfaces changed dramatically, depending on the size of the pattern, incubation temperature and thickness of the brushes. Positively charged poly((2-(methacryloyloxy)ethyl) trimethylammonium chloride) (PMETA) brushes were shown to be promotive for neuron growth. Patterned PMETA brushes surrounded by a poly(ethylene glycol) monolayer guided neuron growth very well. Finally, several types of pH-responsive polymer brushes were used to tailor electrochemical properties of gold electrodes. Cyclic voltammetry of the modified electrodes showed that the redox process of charged electroactive compounds greatly depended on the nature of the brushes and pH of the medium. PE brushes could screen the electrodes against like-charged interferences and increase the selectivity of the electrodes in biosensing.