This paper describes results and challenges in feedback control of microfluidic systems. Results are provided for two representative examples: control of liquid droplets by electrically actuated surface tension forces and steering of many particles at once by micro flow control. Common themes and challenges are outlined based on the authors' research programs and on the results of the March 2004 NSF workshop on 'Control and System Integration of Micro- and Nano-Scale Systems' organized by the author. Copyright (c) 2005 John Wiley & Sons, Ltd.