Traditionally, FreeBSD used a monolithic kernel. The kernel was one large program, supported a fixed list of devices, and in order to change the kernel's behavior, one had to compile and then reboot into a new kernel.

Today, most of the functionality in the FreeBSD kernel is contained in modules which can be dynamically loaded and unloaded from the kernel as necessary. This allows the running kernel to adapt immediately to new hardware or for new functionality to be brought into the kernel. This is known as a modular kernel.

Occasionally, it is still necessary to perform static kernel configuration. Sometimes the needed functionality is so tied to the kernel that it can not be made dynamically loadable. Some security environments prevent the loading and unloading of kernel modules and require that only needed functionality is statically compiled into the kernel.

Building a custom kernel is often a rite of passage for advanced BSD users. This process, while time consuming, can provide benefits to the FreeBSD system. Unlike the GENERIC kernel, which must support a wide range of hardware, a custom kernel can be stripped down to only provide support for that computer's hardware. This has a number of benefits, such as:

Before building a custom kernel, consider the reason for doing so. If there is a need for specific hardware support, it may already exist as a module.

Kernel modules exist in /boot/kernel and may be dynamically loaded into the running kernel using kldload(8). Most kernel drivers have a loadable module and manual page. For example, the ath(4) wireless Ethernet driver has the following information in its manual page:

Alternatively, to load the driver as a module at boot time, place the
following line in loader.conf(5):

    if_ath_load="YES"

Adding if_ath_load="YES" to /boot/loader.conf will load this module dynamically at boot time.

In some cases, there is no associated module in /boot/kernel. This is mostly true for certain subsystems.