A high-speed, large-scale architecture, three-dimensional micro-optical switching system (3-D MOSS) is proposed. A switching network is divided into subnetwork blocks, followed by stacking them to construct a multilayer structure. The interblock waveguide connection is replaced with short-distance vertical optical wiring, that is, optical z connections. Thus, 3D-MOSS, in contrast with conventional planar structures, reduces waveguide cross points, wiring length, and system size. Expected applications are switching for fiber communications, reconfigurable 3-D micro optoelectronic (OE) systems, and so on. 3D-MOSS consists of OE films, in which thin-film high-speed micro-optical switches are embedded. Two critical issues for 3D-MOSS, micro-optical switch and optical z connection, are investigated. The beam propagation method (BPM) calculation shows that waveguide-prism-deflector micro-optical switch (WPD-MOS), in which prism-shaped electrodes are formed on an electro-optic slab waveguide, is suitable for the micro-optical switch. The finite difference time domain method (FDTD)/BPM coupled simulation demonstrates a possibility of low-loss optical z connection in a 4-μm width waveguide-based 3-D micro-optical network. Performance of 3D-MOSS for a 32×32 Banyan network is assessed as follows: system size is 2250 (length)×600 (width)×320 (height) μm3, operation voltage 110 V, switching speed <1 μs, power consumption 520 mW at a switching rate of 3×105/s, and maximum insertion loss 14 dB. This indicates the viability of 3D-MOSS from viewpoints of channel count, switching speed, thermal management, and power budget. A material/cost-saving device integration process "photolithographic packaging with selectively occupied repeated transfer (PL-Pack with SORT)" is briefly described as an example of a fabrication method for 3D-MOSS.