Microbubble transport in fully developed turbulent channel flow is investigated using an Eulerian-Lagrangian approach. The carrier-phase flow is computed using Direct Numerical Simulation (DNS) or Large Eddy Simulation (LES) of the incompressible Navier-Stokes equations. Lagrangian particle tracking is employed for a dispersed phase comprised of small, rigid spheres of negligible density compared to the carrier-phase flow and obeying an equation of motion in which the forces used to predict the motion of the bubble are drag, pressure gradient, and added mass. In general, DNS and LES yield similar predictions of the carrier-phase flow and dispersed-phase properties. The bubble Stokes number is varied over a range for which the dispersed phase essentially follows the carrier flow to larger values for which strong segregation of the microbubbles into coherent vortical structures occurs. In general, simulation results show that microbubble response is not a monotonic function of the Stokes number. The most significant structure in the concentration field occurs for Stokes numbers close to the turbulence timescales in the buffer layer. More than 2/3 of the microbubble population in the buffer layer resides in coherent structures that occupy approximately 1/3 of the computational volume.