At any time, if we do any change in the FreeBSD Kernel like a new feature implemented in the Kernel, or a Kernel module, a new driver included in the Kernel, fixing issues in Kernel etc. it requires building the updated Kernel source files and other dependent modules. You can find a way of building Kernel in a previous post. Here is one more way of building FreeBSD Kernel. Before this step, building world is required. If you want to build a custom kernel, and already have a configuration file, just use KERNCONF=MYKERNEL like this: # cd /usr/src # make buildkernel KERNCONF=MYKERNEL # make installkernel KERNCONF=MYKERNEL Source: FreeBSD Handbook