In aqueous solutions, trehalose possesses a high propensity to form hydrogen bonds with water as well as other trehalose molecules. This hydrogen bonding not only affects water structure but also promotes extensive concentration dependent aggregation of trehalose molecules, which may impact trehalose's role as a protective cosolute to biomacromolecules. To study the association of trehalose in aqueous solutions over a wide concentration range, we used molecular dynamics simulations based on two different force fields, as well as vapor pressure osmometry. By analyzing trehalose cluster size and fractal dimension in simulations, we estimate the cluster percolation threshold at 1.5-2.2 m. Experimentally, trehalose solutions showed positive deviations from ideal van't Hoff's law that grew with concentration. These variations in osmotic pressure can be explained using a simple equation of state that accounts for the repulsive excluded volume interactions between trehalose molecules as well as attractions reflected in sugar clustering. We find that simulations with both applied force fields result in reasonable representations of the solution equation of state. However, in contrast to experiments, the balance between the repulsive and attractive trehalose-trehalose interactions in simulations results in a slightly negative deviation from ideality, probably due to the moderately overaggregative nature of the force fields used.