Advancement towards efficiency improvement of the anaerobic digestion process (AD) is often quoted to be dependent on two major key subjects of research (1) an in depth understanding of the structure and dynamics of the microbial communities/populations involved in the process and (2) the development of on-line monitoring tools to better predict process dysfunction occurring because of inadequate organic loading rate (OLR). Although a lot of information has been generated about the basic metabolism in different types of anaerobic reactors, little is known about the microbes responsible for the anaerobic digestion processes. Only a few percentages of the Eubacteria and Archaea have so far been isolated and characterized. The dynamics and interactions between Eubacteria and Archaea are currently considered as key subjects of research towards the improvement of the AD process. Knowledge on the ecology of AD and on the dynamics of the microbial populations and their structure could provide valuable information on unexplained and unpredictable failures or malfunctions of the AD process. To understand the ecology of the microbial communities involved in AD three major points need to be addressed: (1) identification of microorganisms involved in AD, (2) quantification of their abundance, and (3) identification and quantification of their activity. The fast and promising advances in molecular microbial ecology are providing new opportunities for the characterization and direct identification (gene sequencing, RFPL, DGGE, nucleic acid probes e.g. FISH, PCR), and quantification (Real Time-PCR) of microorganisms involved in AD. The major objectives of the GASPOP project are therefore (1) to better understand the ecology and diversity/dynamics of the microbial communities involved in the AD process when exposed to shifts in the operational parameters, (2) to compare the influence of the reactor type (continuously stirred tank reactors (CSTR) versus plug flow reactors (PFR)), and the temperature (37 versus 55°C) on the proportion of fermentative bacteria/acetogens/methanogens, and (3) to evaluate the consequences of the reactor type on the process efficiency when submitted to increasing organic loading rates (OLR). Additionally, while monitoring the metabolism of the reactors and the biogas production and quality, (4) an integrated “omics” study will be conducted to extract and quantitate metabolites and key enzymes involved in the process to possibly pave the way towards the production of molecular markers and probes aiming at the rapid diagnostic of the process status.