It is well established in the scientific literature that only a small fraction of microorganisms can be cultured by conventional microbiology methods. The ever cheaper and faster DNA sequencing methods, together with advances in bioinformatics, have improved our understanding of the structure and functional behavior of microbial communities in many complex environments. However, the metagenomics approach alone cannot elucidate the functionality of all microorganisms, because a vast number of potentially new genes have no homologs in public databases. Metatranscriptomics and metaproteomics are approaches based on different techniques and have recently emerged as promising techniques to describe microbial activities within a given environment at the molecular level. In this review, we will discuss current developments and applications of metagenomics, metatranscriptomics and metaproteomics, and their limitations in the study of microbial communities. The combined analysis of genes, mRNA and protein in complex microbial environments will be key to identify novel biological molecules for biotechnological purposes.