Abstract. – Many marine sponges live in a highly specific and symbiotic relationship with phylogenetically complex microbial consortia. This complexity and the resistance of most microbes to cultivation make the study of these symbioses a challenging endeavor. The recent implementation of 16S rRNA gene based methods for microbial community characterization as well as metagenomic approaches for functional genomic studies greatly enhanced our current understanding of sponge-microbe associations. In this review, we consider the following fields as new frontiers in this unique association: (i) microbial diversity including bacteria as well as archaea, (ii) microbial primary and secondary metabolism and their ecological relevance for the symbiosis, (iii) vertical microbial transmission, and (iv) bioactive compound discovery from sponges and their associated microbes. This review highlights recent important results in these fields and presents future perspectives in the promising area of sponge-microbe research.