Mixed-dimensional van der Waals (vdW) heterostructures combine two-dimensional (2D) materials with non-2D materials, enabling new device applications. This review discusses the physics, fabrication, and applications of these heterostructures in electronics, optoelectronics, and photovoltaics. The integration of 2D materials with 0D, 1D, and 3D materials allows for tunable electronic and optical properties. For example, 2D materials like graphene and transition metal dichalcogenides (TMDCs) can be combined with quantum dots, nanotubes, and semiconductors to create heterostructures with enhanced performance. These structures offer advantages in charge transport, band alignment, and device functionality. In logic devices, mixed-dimensional heterostructures enable high-speed, gate-tunable transistors and diodes. For instance, vertical field-effect transistors (v-FETs) using 2D materials and 3D semiconductors show improved performance. Similarly, p-n heterojunctions based on 2D materials and organic semiconductors demonstrate tunable rectification and anti-ambipolar transfer characteristics. These devices have potential applications in analog circuits and wireless communication. In photodetectors and photovoltaics, mixed-dimensional heterostructures enhance optical absorption and carrier collection. For example, graphene-silicon heterojunctions and TMDC-based photodetectors show high responsivity and efficiency. The integration of nanophotonic elements with these heterostructures further improves light trapping and optical absorption. In photovoltaics, graphene-silicon devices with micropillar arrays achieve higher power conversion efficiencies. Light-emitting devices based on mixed-dimensional heterostructures, such as GaN/InGaN LEDs with graphene buffers, demonstrate improved performance and heat dissipation. These devices leverage the unique properties of 2D materials to enhance radiative recombination and emission efficiency. Additionally, surface passivation and encapsulation techniques are crucial for minimizing non-radiative recombination in these devices. The integration of 2D materials with other materials presents significant opportunities for future semiconductor technologies. However, challenges remain in achieving high-quality, large-area synthesis and integration. Despite these challenges, mixed-dimensional vdW heterostructures show promise for both fundamental research and practical applications in electronics, optoelectronics, and photovoltaics.Mixed-dimensional van der Waals (vdW) heterostructures combine two-dimensional (2D) materials with non-2D materials, enabling new device applications. This review discusses the physics, fabrication, and applications of these heterostructures in electronics, optoelectronics, and photovoltaics. The integration of 2D materials with 0D, 1D, and 3D materials allows for tunable electronic and optical properties. For example, 2D materials like graphene and transition metal dichalcogenides (TMDCs) can be combined with quantum dots, nanotubes, and semiconductors to create heterostructures with enhanced performance. These structures offer advantages in charge transport, band alignment, and device functionality. In logic devices, mixed-dimensional heterostructures enable high-speed, gate-tunable transistors and diodes. For instance, vertical field-effect transistors (v-FETs) using 2D materials and 3D semiconductors show improved performance. Similarly, p-n heterojunctions based on 2D materials and organic semiconductors demonstrate tunable rectification and anti-ambipolar transfer characteristics. These devices have potential applications in analog circuits and wireless communication. In photodetectors and photovoltaics, mixed-dimensional heterostructures enhance optical absorption and carrier collection. For example, graphene-silicon heterojunctions and TMDC-based photodetectors show high responsivity and efficiency. The integration of nanophotonic elements with these heterostructures further improves light trapping and optical absorption. In photovoltaics, graphene-silicon devices with micropillar arrays achieve higher power conversion efficiencies. Light-emitting devices based on mixed-dimensional heterostructures, such as GaN/InGaN LEDs with graphene buffers, demonstrate improved performance and heat dissipation. These devices leverage the unique properties of 2D materials to enhance radiative recombination and emission efficiency. Additionally, surface passivation and encapsulation techniques are crucial for minimizing non-radiative recombination in these devices. The integration of 2D materials with other materials presents significant opportunities for future semiconductor technologies. However, challenges remain in achieving high-quality, large-area synthesis and integration. Despite these challenges, mixed-dimensional vdW heterostructures show promise for both fundamental research and practical applications in electronics, optoelectronics, and photovoltaics.