The goal of this research is to create custom analog VLSI semiconductor technology to provide a lightweight, compact implementation of a data acquisition system for ambulatory EEG recording using EGFs Geodesic Sensor Net, an electrode system noted for its comfort and ease of installation even with very high channel counts (to 256 electrodes). Recent high-performance laboratory EEG amplifier designs are providing full DC bandwidth capability, by using a single 24-bit delta-sigma A/D converter for each channel. This capability is especially important in epilepsy studies, for prompt recovery from artifact situations (such as might easily occur during seizures), as well as because very low frequency phenomena may be clinically significant. By combining multiple input instrumentation amplifier channels and delta-sigma modulators on a single chip, and integrating the digital filtering and signal processing functions of multiple delta-sigma converters on a single FPGA (Field Programmable Gate Array) we can implement a full-bandwidth system design with much smaller overall size and reduced power consumption, compared to existing systems. Ambulatory systems are gaining increasing acceptance in EEG for epilepsy assessment, because of lower costs of outpatient recording and better opportunities to capture epileptiform events, with longer recording times in a variety of settings. One recent study of source localization of epileptic foci has shown that dense array EEG provides substantial improvements in source estimation, while a study in our own lab indicates that dense array also facilitates improved rates of interictal spike detection. Thus, any conventional (low density) research or clinical strategy (including efforts directed at seizure prediction and suppression) risks substantially sub-optimal result. In addition to the benefits for fully ambulatory applications, our proposed system will also lead to greater acceptability of inpatient high-density recordings, because of improved patient mobility and comfort of an un-tethered data acquisition mode. The system will incorporate local disk storage or a wireless data link for continuous downloading to a base station.