Monolithic 3D integration is the ultimate approach in 3D-ICs as it provides high-density and submicron vertical interconnects and hence transistor level integration. Here, high-quality Si layer formation at a low temperature is a key challenge. We review our recent achievements in monolithic 3D-ICs based on single-grain Si TFTs that are fabricated inside a single-grain with a low-temperature process. With the mu-Czochralski process based on a pulsed-laser crystallization, Si grains with a diameter of 6 mu m are successfully formed on predetermined positions. Single-grain (SG) Si TFTs are fabricated inside the single-grain with mobility for electron and holes of 600 cm(2)/V s and 200 cm(2)/V s, respectively. Two layers of the SG Si TFT were vertically stacked and successfully implemented into CMOS inverter, 3D 6T-SRAM and single-grain lateral PIN photo-diode with in-pixel amplifier. Those results indicate that the SG TFrs are attractive for use in monolithic 3D-ICs on an arbitrary substrate including a glass and even a plastic for applications such as ultra-high-density memories, logic-to-logic integration, CPU integrated display, and high-definition image sensor for artificial retina. (C) 2011 Elsevier Ltd. All rights reserved.