This paper concentrates on recent developments in microsensors and microbiosensors for the possible applications in visual prostheses, especially retinal prosthetic devices. A brief introduction on the developments of visual prosthesis will be presented. The importance for in-vivo pH measurements as well as the need for an implantable pH sensor will be demonstrated. Electrochemical biosensors developed for sensitive measurements of glucose and L-glutamate, a known neurotransmitter in the retina and brain will be reviewed. Novel electrode materials such as chemically modified thin-film diamond in applications for implantable biosensors will be shown. The challenges in the development of chronic implantable sensor systems, especially using MEMS technology for medical implants, will be discussed.