Gigascale integration: is the sky the limit?

Early 21st century opportunities for Gigascale integration (GSI) will be governed by a hierarchy of theoretical and practical limits whose five levels can be labeled as: 1) fundamental, 2) material, 3) device, 4) circuit, and 5) system. This article reviews recent enhancements of this hierarchy and identifies those critical limits that appear to present the most formidable challenges to continued progress toward GSI.