Capacitive sensors are widely used for proximity applications ranging from touch-sensitive screens to sensors for classifying human proximity. It is a well-established field with contributions from many researchers. In this chapter, an interdigitated capacitive proximity sensor to address an unsolved problem of children from being left behind in hot cars. The sensor presented is integrated into the infant seat. It works by utilizing the large relative permittivity of the human body. The existing mechanisms utilize the weight of the child/infant or a mechanical switch under the seat to detect the presence and fail to meet the functionality requirements sufficiently. As the presented one is not relying on such parameters and it only requires the proximity, the overall reliability that can be achieved is high. The electrode structure, together with the measurement scheme utilizes the coupling and shielding effects of the electric field. The electrode structure is a combination of a planar two-electrode scheme and interdigitated electrodes. The chapter presents a brief note on the state of the art, a potential solution, and test results from a prototype developed and evaluated in a car.