Hydrogen and the related technologies are seen as potential tools for the energy transition, forming modern hydrogen-based multi-energy systems (HyMES) with renewable energy sources and the electrical grid. This chapter studies the up-to-now state of the art of Fuel Cells (FC), Electrolyzers and their potential application in Hydrogen Energy Systems Integration (HESI), while having the ability to perform Power-to-Hydrogen-to-Power (P2H2P). In the P2H2P systems, the role of power converters, hydrogen storage and ancillary equipment for a grid-connected application are of importance, as well as the main controls of the system determining the power absorbed or supported by the hydrogen units in the system. In this chapter, readers will gain insights into the comprehensive design and operation of P2H2P systems, introducing FC, Hydrogen Turbines and electrolyzers as a complementary resource to the grid. The presented analysis provides insights into the role of hydrogen technologies within the context of the energy transition, emphasizing their capcaity to enhance grid stability and reliability.