Quantum chemical (QM), classical molecular dynamics (MD), and Car-Parrinello (CP-MD) studies are reported for 18-crown-6 (18C6) and its first 18C6(H2O)(n) hydrates, focusing on the D-3d and C-i forms of the crown. They reveal the importance of dynamics and the surrounding medium on its conformational and hydrogen-bonding properties. In the gas phase, the two forms of the free crown are found to be quasi-isoenergetic at several computational levels, but during CP-MD simulations, the D-3d form is more mobile than Ci and undergoes conformational changes in such a way as "to fill its own cavity". Among several forms of the monohydrate, the one with a D-3d-type crown and a bridging water molecule is most stable. Along its 10-ps CP-MD trajectory, the H2O molecule undergoes a "merry-go-round" dynamics, exchanging between the three "top" oxygens of the deformed crown, and is thus more often instantaneously monodentate than bidentate. The static and dynamic results of different forms of the mono- and dihydrates confirm the importance of dynamic bridging coordination to 18C6. These results solve the apparent contradiction between IR spectroscopic results in humid CCl4 or supercritical-CO2 solutions that hint at an equilibrium between monodentate and bidentate hydrogen bonds, whereas in other humid phases (solid-state structures, liquid hydrates, simulated aqueous solutions), the hydrated crown is always D-3d-like and the first coordinated H2O molecules are bridging.