This paper describes a simple, versatile method of generating gradients in composition in solution or on surfaces using microfluidic systems. This method is based on controlled diffusive mixing of species in solutions that are flowing laminarly, at low Reynolds number, inside a network of microchannels. We demonstrate the use of this procedure to generate (1) gradients in the compositions of solutions, measured directly by colorimetric assays and (2) gradients in topography of the surfaces produced by generating concentration gradients of etching reagents, and then using these gradients to etch profiles into the substrate. The lateral dimensions of the gradients examined here, which went from 350 to 900 mum, are determined by the width of the microchannels. Gradients of different size, resolution, arid shape have been generated using this method. The shape of the gradients can be changed continuously (dynamic gradients) by varying the relative flow velocities of the input streams of fluids. The method is experimentally simple and highly adaptable, and requires no special equipment except for an elastomeric relief structure that can be readily prepared by rapid prototyping. This technique provides a new platform with which to study phenomena that depend on gradients in concentration, especially dynamic phenomena in cell biology (chemotaxis and haptotaxis) and surface chemistry (nucleation and growth of crystals, etching, and Marangoni effects).