Localized electrochemical deposition (LECD) is an emerging technology for inexpensive and effective fabrication of high-aspect-ratio microstructure of diverse materials. Microradiology with coherent X-rays enabled study of this process in real-time. This led to the discovery of a fundamental role of the microelectrode-structure distance: for short distances the deposition rate increases dramatically but the product becomes porous. This role is explained qualitatively with the interplay between metal-ion diffusion and migration in the deposition process. (C) 2004 The Electrochemical Society.