Abstract: Freak waves, alternatively called rogue waves or giant waves, are exceptionally large, steep, and asymmetric waves whose heights usually exceed by 2.2 times the significant wave height. They have also been described as "holes in the sea", "walls of waters", or three sisters! These waves have been long known to be notorious hazards to navigation vessels and marine structures. Many sinister marine episodes and their devastating impacts have prompted a great interest in freak waves. With little warning, freak waves often mysteriously occur as transient giant waves from wave groups in random coastal and open seas. While statistical methods are widely employed in examining the occurrence of such extreme sea conditions, it is still unclear whether freak waves are rare realization of a typical population or typical realization of a rare population. Likewise, it is unclear the physical mechanisms of freak wave formation and its characteristics. In this talk, we will report the recent laboratory measurements on limiting freak waves on currents. It is found that wave group structure is critical to determine the formation and the geometric properties of freak waves. Strong opposing currents inducing partial wave-blocking can significantly promote the freak waves, which occur often in the Great Lakes and Oceans.