Despite the many ergonomic advantages of eccrine perspiration (sweat) compared to other possible biofluids (particularly in “wearable” devices), sweat remains an underrepresented source of biomarker analytes compared to the established biofluids blood, urine, and saliva. Upon closer comparison to other non-invasive biofluids, the advantages may even extend beyond ergonomics: sweat might provide superior analyte information. A number of challenges, however, have historically kept sweat from its place in the pantheon of clinical samples. These challenges include very low sample volumes (nL to µL), unknown concentration due to evaporation, filtration and dilution of large analytes, mixing of old and new sweat, and the potential for contamination from the skin surface. More recently, rapid progress in “wearable” sweat sampling and sensing devices has resolved several of the historical challenges. However, this recent progress has also been limited to high concentration analytes (µM to mM) sampled at high sweat rates (>1 nL/min/gland, e.g. athletics). Progress will be much more challenging as sweat biosensing moves towards use with sedentary users (low sweat rates or not sweating at all) and/or towards low concentration analytes (pM to nM). Addressing these unresolved challenges will require significant advances in sweat stimulation, sample collection efficiency, compact sensors, and likely more. Fortunately, none of the remaining challenges appear to be fundamentally blocking, and scientific and engineering innovations have the opportunity to enable broader application of sweat biosensing technology.