A gel polymer electrolyte (GPE) is a liquid electrolyte (LE) entrapped by a small amount of polymer network less than several wt%, which is characterized by properties between those of liquid and solid electrolytes in terms of the ionic conductivity and physical phase. Electrolyte leakage and flammability, demerits of liquid electrolytes, can be mitigated by using GPEs in electrochemical cells. However, the contact problems between GPEs and porous electrodes are challenging because it is difficult to incorporate GPEs into the pores and voids of electrodes. Herein, the focus is on GPEs that are gelated in situ within cells instead of covering comprehensive studies of GPEs. A mixture of LE and monomer or polymer in a liquid phase is introduced into a pre‐assembled cell without electrolyte, followed by thermal gelation based on physical gelation, monomer polymerization, or polymer cross‐linking. Therefore, GPEs are formed omnipresent in cells, covering the pores of electrode material particles, and even the pores of separators. As a result, different from ex situ formed GPEs, the in situ GPEs have no electrode/electrolyte contact problems. Functional GPEs are introduced as a more advanced form of GPEs, improving lithium‐ion transference number or capturing transition metals released from electrode materials. Gel polymer electrolytes characterized by in situ gelation are presented. Different from ex situ gelated GPEs outside of electrochemical cells, in situ GPEs are formed omnipresent within cells covering the pores of the electrode material particles, and even the pores of the separators. The merits of in situ GPEs are demonstrated in electrochemical energy systems such as lithium‐ion batteries and lithium–sulfur batteries.