What is the difference between include_directories and target_include_directories in CMake?

include_directories(x/y) affects directory scope. All targets in this CMakeList, as well as those in all subdirectories added after the point of its call, will have the path x/y added to their include path.

target_include_directories(t x/y) has target scope—it adds x/y to the include path for target t.

You want the former one if all of your targets use the include directories in question. You want the latter one if the path is specific to a target, or if you want finer control of the path's visibility. The latter comes from the fact that target_include_directories() supports the PRIVATE, PUBLIC, and INTERFACE qualifiers.

Beside what Angew's answer correctly says, another very important difference between include_directories and target_include_directories is that, when used with PUBLIC or INTERFACE, the latter populate the INTERFACE_INCLUDE_DIRECTORIES property of the target. This property is useful when another target uses target_link_libraries to link to the original target, as the linking target will have automatically those include directories added. See example.

This important feature is pretty well hidden in the documentation: target_include_directories mention populating INTERFACE_INCLUDE_DIRECTORIES, whose documentation says:

When target dependencies are specified using target_link_libraries(), CMake will read this property from all target dependencies to determine the build properties of the consumer.

As @Angew said, the very difference is :

1, include_directories() is accessible for all the files in the source-tree 2, target_include_directories() is-only accessible for a specific target when compile.