Two problems that we encounter in the structure formation of polymeric systems are reviewed. One is the dynamics of phase separation of polymer blends and the other is the intramolecular structure formation of associating polymers. In the case of phase separation of polymer blends, we review the model and the simulation method that is suitable for large-scale computer simulations of phase separation of binary fluid mixtures. We also show that simulation results are in quantitative agreement with experimental results of polymer blends. In the case of associating polymers, we treat the intramolecular structure formation in single associating polymers. In order to study the structure formation in polymers with strong attractive interactions, we employ the multicanonical simulation method. We show that a two-step intramolecular conformational transition occurs in periodic associating polymers where associative groups are periodically placed along the chain. With decreasing the temperature, a transition from random-coil conformations to micelles occurs and multiple flower-type micelles are formed via the transition. The number of the associative groups forming a micelle core is limited by the excluded volume effect of loop chains around micelle cores. By this effect, two intramolecular micelles are formed for long polymer chains with 60 bonds via the coil-to-micelle transition. By further decreasing the temperature, we find that another transition, i.e., a micelle-to-micelle transition takes place. At this transition point, the two intramolecular micelles merge into one micelle. (c) 2005 Elsevier Ltd. All rights reserved.