Using Monte Carlo simulations, we show that the range of repulsive head-head interactions has to be greater than or equal to the range of attractive tail-tail interactions to facilitate micellization in model surfactants. These results represent the first quantitative verification of a well accepted conjecture that the range of repulsions has to be much larger than the range of attractions for micellization. to occur. Our results verify this conjecture but add to it by suggesting that micellization can even occur when the ranges of the two interactions are comparable. Studies of micelle structure show that we proceed from macroscopic phase separation, when the attractions are longer range than the head-head repulsions, to extended, wormlike micelles, which form when the two interactions have comparable range, to compact spheres, when the attractions are short range. These results provide new insights into the factors affecting the sizes and shapes of micelles and suggest that tuning the relative ranges of the repulsive and attractive interactions [e.g., through the addition of salt to a solution of ionic surfactant] can provide a facile means of controlling these structures.