The use of hydrogels for novel micromechanical sensors and actuators has been the focus of the Research Training Group (RTG) 1865 "Hydrogel-Based Microsystems", which was funded by the German Research Foundation from 2013 to 2023. The RTG pursued two objectives: (i) researching the scientific basis for novel and application-specific hydrogel materials with properties that are essential for their use in sensors and actuators, and (ii) applying these specially tailored materials to specific microsystems. To achieve this goal, a number of challenges still need to be overcome that currently hinder or even prevent the use of hydrogels in technical applications. On the other hand, the development of improved materials and manufacturing processes is necessary to make hydrogels suitable for practical applications (relevant functionality; high sensitivity, selectivity and long-term stability; fast response time). This article focuses on research findings of the RTG with regard to materials, sensor principles, measurement methods, and special applications in which the use of hydrogels promises innovative solutions. This applies, for example, to tailored hydrogels for chemical and biochemical sensors, pressure-compensated hydrogel sensors with advanced long-term stability, piezoresistive and plasmonic transducers to sense the hydrogel swelling, and sensor switches.