In this work, the formation of micelle and gel from symmetric ABA triblock copolymers in selective solvents is followed using the Monte Carlo technique. The solvent is taken to be athermal for the middle block and poor for the end blocks. The effects of the block size and solubility of the end block A on system properties are explored with a focus on the critical micelle concentration (CMC) and critical gel concentration (phi(gel)*). The relevant micellar and/or network parameters, including the weight-average aggregation number, number density, and functionality are obtained. Copolymer chain population distributions of the elastically active chains (bridges) and the elastically inactive chains (loops, free, and dangling chains) are calculated. Both the CMC and phi(gel)* are strongly dependent on the degree of incompatibility ((2N(A))beta epsilon, a variable which shows the combined effects of both the size and the solvent insolubility of the end block A) but only weakly dependent on the middle block size. As incompatibility increases, the CMC shows a slower decrease than the exponential decay predicted by Leibler et al. (J. Chem. Phys. 1983, 79, 3550) for diblock copolymers at high incompatibility. The dependence of the CMC and phi(gel)* on incompatibility appear to fit power-law type expressions, y similar to x(-a), where y is either the CMC or phi(gel)* and x is a scaling variable (2N(A))beta epsilon. The scaling exponent a is approximately 0.4 for the CMC and is in the range 0.1-0.2 for phi(gel)*. The phase diagram appears complex, containing four regions, exhibiting homogenity, micellization, gel formation, and possible precipitation.