This paper reports the development of flexible and elastic electronic devices that are mainly composed of functional organic materials and brain‑activity monitoring systems. We developed two different brain‑activity monitoring systems. One is a sheet-type brain-wave sensor system (electroencephalogram: EEG) that can monitor brain waves by simply attaching the sensor to the forehead. The other is an intracranial (brain implant) brain‑state sensory network (Electrocorticogram: ECoG; Local field potential: LFP) developed to understand higher-order cerebration in primates.   Our laboratory has developed processes for elaborately laminating nanomaterials on ultrathin or flexible thin rubber films [1–5]. We have been conducting research and development of ultra‑flexible elastic electronics using novel techniques [6–10]. We have succeeded in developing a system for monitoring biopotentials by combining four modalities: (1) a flexible/elastic and biocompatible electrode with high electrical conductivity; (2) a flexible thin-film amplifier to amplify very weak biosignals; (3) a Si-LSI platform with wireless communication; and (4) a signal‑processing technique for visualizing signals in real time. The developed system is a sheet-type wireless system that is less than 5‑mm thick weighs less than 20 g. Regardless of this small size, its measurement accuracy is as high as 0.1 μV; essentially, it can monitor very weak brain waves. Using this system, we developed a brain-wave sensory patch and an intercranial brain-state sensor.   The patched brain-wave sensor has a measurement accuracy comparable to that of sophisticated medical equipment. In addition, brain waves can be monitored by simply attaching the sensor onto the forehead; thus, it has been used not only in medical applications but also in applications such as the development of brain‑wave products, measurement of the quality of sleep, monitoring of brain waves during sport activities, and easy monitoring of cerebration at home.   The brain‑state sensory implant enables comprehensive monitoring of brain activities from the cerebral cortex to the deep brain and is expected to help understand high-order cerebration. The flexible, thin, and soft sheet-type multichannel electrodes can be widely distributed without damaging the cerebral cortex. The surfaces of the ECoG and LFP electrodes are completely covered with biocompatible conductive gel, which enables us to simultaneously measure comprehensive brain activity during moving for long periods, up to several months.   This paper elaborates the results of the brain‑monitoring system realized by imperceptible electronics. The novel system is a fusion of flexible and stretchable electrical components characterized by the softness and lightness of functional organic materials and advanced Si-LSI technology.