Hydrogels are hydrophilic, highly water swellable polymer networks capable of converting chemical energy into mechanical energy and vice versa. They can be tailored regarding their chemical nature and physical structure, sensitiveness to external stimuli and biocompatibility; they can be formed in various structures and integrated into (micro-)systems. Accordingly, over the last decade, these materials have gained considerable recognition as valuable tool for sensors and in diagnostics. This article reviews the use of hydrogels in sensor development with focus on recent efforts in the application of stimuli responsive hydrogels as sensors, hydrogels as suitable matrices in which the sensing (bio-)molecules are embedded and hydrogels for modification and protection of sensor surfaces. In the first part of the review, both sensors and hydrogels are defined and a basic theoretical overview of hydrogels and their behavior is given. Subsequent chapters focus on hydrogels in physicochemical and biochemical sensing mechanisms with a primary emphasis on the hydrogels as such and the applied sensing mechanism. Finally, the review is concluded by a summary and discussion including an outlook on future perspectives for hydrogels in sensing applications. (C) 2012 Elsevier Ltd. All rights reserved.