Red Hat Enterprise Linux is Red Hat's premium Linux offering, being targeted solely at business and mission-critical environments. Those of you who are familiar with RHEL will know that the kernel version never changes, in order to maintain compatibility at the API and ABI level (Application Programming Interface and Application Binary Interface). Therefore, the 2.6.18 kernel that Red Hat ships with RHEL 5 will only have security fixes and relevant backports added to it. Yes, you read that right - Red Hat will backport key parts of later kernels into 2.6.18. RHEL 3 also did this, except that then Red Hat backported key features from the 2.6 kernel to the 2.4 kernel, actually taking a fair amount of chastisement in the meantime from competitors who had moved on. Red Hat's expertise in this area means that you are in the safest hands possible, especially when you consider that Red Hat developers are directly responsible for 12.8% of the patches made to the 2.6.20 kernel - the next largest contributor was IBM, with 7.4% (LWN: Who Wrote 2.6.20).