Not only muscle contraction, but also most cell movements depend on myosin - actin interaction using ATP. Many components of the contraction machinery are involved in the efficient coupling of energy source and force development. Among these, I have focused on myosin light chain kinase (MLCK) in this review. MLCK phosphorylates myosin regulatory light chain and controls all 3 types of muscle contraction: skeletal muscle, smooth muscle, and cardiac muscle. However, each muscle has specific MLCK and the role of MLCK in each muscle is different. This difference explains the specific role of each muscle in vivo and contributes to the activity of various force development in different ways in each tissue. Therefore, I also review the differences in the connection between each MLCK and muscle contraction in the 3 muscle types. (Circ J 2009; 73: 208 - 213)