There are, at the moment, five revisions of the SSE instruction set (and hence of the technology), which are as follows:

• SSE: This was introduced in 1999 and contained the initial design of the technology and instructions thereof
• SSE2: This revision came out with Pentium 4 and brought 144 new instructions with it
• SSE3: Although only 13 new instructions were added with SSE3, they introduced the ability to perform the so-called "horizontal" operations (operations performed on a single register)
• SSSE3: This introduced 16 new instructions, including the instructions for horizontal integer operations
• SSE4: This brought another 54 instructions, thus making life significantly easier for developers