Protein folding, binding, catalytic activity and molecular recognition all involve molecular movements, with varying extents. The molecular movements are brought upon via flexible regions. Stemming from sequence, a fine tuning of electrostatic and hydrophobic properties of the protein fold determine flexible and rigid regions. Studies show flexible regions usually lack electrostatic interactions, such as salt-bridges and hydrogen-bonds, while the rigid regions often have larger number of such electrostatic interactions. Protein flexible regions are not simply an outcome of looser packing or instability, rather they are evolutionally selected. In this review article we highlight the significance of protein flexibilities in folding, binding and function, and their structural and thermodynamic determinants. Our electrostatic calculations and molecular dynamic simulations on an antibody-antigen complex further illustrate the importance of protein flexibilities in binding and function.