Hydrogels have been extensively studied for biomedical applications such as tissue engineering and drug delivery. The physicochemical, biomechanical and biological properties of hydrogels based on various synthetic or natural polymers have been characterized, and numerous recipes to optimize these properties have been established. However, relatively little research has been conducted in relation to the optimization of their optical properties. Here, I will talk about the recent development of hydrogel-based optical devices with emphasis on light-guiding waveguides. Several wearable, implantable, biodegradable, and cell-encapsulating devices will be presented, along with proof-of-concept demonstrations of their medical applications, solving the limitations of conventional optical devices and conventional approaches.