Chemomechanical polymers can act as sensor and activator within the same unit, without the need for any transducers or power supplies. The implementation of supramolecular binding sites in hydrogels allows the design of smart chemoresponsive materials for a manifold of external chemical stimuli, including for instance metal ions, nucleotides, amino acids, peptides, or carbohydrates. The principles ruling the volume changes of such materials are exemplified with a large range of effector compounds and several gel polymer structures. Continuous water content changes are found to be the major driving force for the observed expansions and contractions, without indication of discontinuous phase transitions. A particularly interesting feature is the often observed cooperativity between different effector substances. Such systems represent logical gate functions, and allow for instance the use of effector molecules such as peptides, which by themselves are inactive. Chemomechanical gels hold promise for several applications, in particular for new drug release systems which bear sensing and release units within the same particle. Miniaturization of the gel particles allows the enhancement of the speed and sensitivity of the response by orders of magnitude.