Species are fundamental units used to describe and interpret nature. Molecular delimitation methods have shown that many species cannot be diagnosed using morphological traits. We suggest that cryptic species represent an opportunity to progress Linnean taxonomy,. We examine what can be learned from sympatric cryptic species pairs that occupy the same habitat. Their sympatry is possible under two conditions, 1) reproductive isolation, 2) the effects of interspecific competition on population growth must be neutralised. Understanding the mechanisms that maintain species differences and integrating this understanding into taxonomy will help to delineate species boundaries. We first examine the mechanisms of cryptic species origin. Then we review some well-documented cases of reproductive isolation between cryptic species, focusing on prezygotic isolation mechanisms mediated by premating recognition and communication. We follow with examples of co-occurring cryptic species, focusing on mechanisms of coexistence and ecological niche differentiation. Both mate recognition and niche differentiation are grounded in the sensory worlds that animals experience. Sensory ecology provides tools to explore hitherto hidden diversity and so identify, misunderstood and unprotected in this time of rapid anthropogenic global change. We argue that the field of sensory ecology has a potential for improving taxonomy