In this thesis, we study two fundamental problems in the theory and experiment of interfaces in simple and complex fluids. The first problem concerns the critical behavior of the thickness of interface of simple fluids in two dimensions of space. We study this problem using both theory and computer simulations. In our theoretical study of this problem, the equilibrium interface is assumed, in accord with current ideas, to consist of an intrinsic interface of the non-classical van der Waals type broadened by capillary wave fluctuations. It is shown that the interfacial thickness exhibits a crossover with change of critical exponent from capillary wave behavior at low temperatures to intrinsic structure behavior at temperatures extremely close to the critical temperature. The location of the crossover temperature is determined by a critical amplitude ratio. In our computer simulation study of this problem, the interfacial thickness between two two-dimensional Lennard-Jones fluid phases is determined by using the method of molecular dynamics. Within statistical uncertainty, the results for the interfacial thickness are found to be consistent with the prediction of the non-classical van der Waals theory of the intrinsic interface. The second problem studied in this thesis concerns the wetting behavior of three-component surfactant systems. In a three-phase equilibrium system, the middle phase either wets or does not wet the interface between the upper and lower phases. In this work, we perform a systematic experimental search for wetting transitions between wetting and nonwetting behaviors in three-component systems water/n-alkane/C\sbiE\sbj, where C\sbiE\sbj denotes the surfactant polyoxyethylene alcohol C\sbiH\sb2i+1(OC\sb2H\sb4)\sbjOH. It is found that two systems, water/n-hexadecane/C\sb6E\sb2 and water/n-octadecane/C\sb6E\sb2, exhibit a wetting transition lying at least 10\sp∘C below the upper critical temperature, and one system, water/n-tetradecane/C\sb6E\sb2, exhibits a wetting transition lying 4.3\sp∘C below the upper critical temperature.