H.323 is the ITU-T teleconferencing protocol suite (which uses the H.261 and H.263 video codecs, the GSM G.711, G.721, and G.723 audio codecs, and a slew of other sub-norms).

Its adoption was stalled mostly due to idiocy on the part of vendors, which tried to extract a premium from the initial implementations. When Microsoft implemented it in NetMeeting, it practically killed off the client software market segment. As a result, clients for other platforms never took off.

The network protocols are also notably fiddly (both in terms of interconnection to the fixed/3G world via MCUs and on top of TCP/IP) and troublesome in today's NAT-oriented world, which doesn't help its adoption either.

H.323's niche market is currently being absorbed by SIP-based video conferencing solutions.