Metal-organic frameworks (MOFs) are porous crystalline materials with a high tunability. To improve the functionality of the original frameworks, several strategies, such as the use of different metal cations and organic ligands and post-synthetic modification, have been developed, enabling the use of MOFs in numerous practical applications in various fields. Recently, another approach, i.e., MOF-on-MOF architecturing, has been actively studied by combining two or more MOFs into a composite. MOF-on-MOF materials not only possess the intrinsic properties of each MOF but also exhibit unprecedented synergism within a single system, resulting in a considerable potential for various applications. This review summarizes the interesting areas of application of MOF-on-MOF architectures into three categories: separation, catalysis, and sensing. In particular, the synergism occurring within such MOF-on-MOF architectures is discussed.