Uncovering and describing biodiversity is fundamental to advancing both scientific understanding and public awareness of nature. While many biodiversity facets are visible and even in the spotlight, including those biodiversity features we encounter on a daily basis, much of the world’s biodiversity remains hidden. This review addresses the critical questions - What dimensions constitute hidden biodiversity? Why does such a large fraction remain hidden, and what factors contribute to these knowledge gaps? Moreover, why is it important to unveil these hidden facets? To address these questions, we (i) dissect the various dimensions of hidden biodiversity, including the taxonomic, spatio-temporal, genetic, functional, social, and methodological dimensions, and (ii) identify prevalent knowledge gaps. Moreover, we (iii) highlight possible ways forward on how to deal with hidden biodiversity, and (iv) conclude with the proposal of guiding principles to shape future research in this field.