Using laminar flow in microfluidic systems, we have been able to generate complex gradients. The gradient generators are designed and fabricated using soft lithography. The basic principle of operation involves splitting and recombining streams of liquid in microchannels; diffusion within these channels accomplishes mixing. The design of the channel system controls the structure of the gradient produced. These gradients will be useful in a range of applications in cell biology, especially in experiments that ask what the cell senses in its environment.