Wearable systems show considerable promise in monitoring and assessing the real-time performance of athletes, the health status of patients, or the general well-being of interested users. Most wearables today focus on monitoring physical parameters (e.g., activity, respiration rate, etc.), or electrophysiology (e.g., ECG, EEG, etc.). In order to augment the richness of collected data, the next-generation of wearables will also be capable of monitoring underlying chemical homeostatis of the user, for example through measurement of glucose in interstitial fluid, lactate in saliva, or electrolytes in sweat. This paper discusses the challenges of building wearable chemical biosensors, including biosensor functionalization, flexible material engineering, bioelectronic integration, and data analytics.