Landfill bioreactors (LBRs) with management of leachate and biogas have presented numerous advantages such as accelerated stabilization of solid wastes, reduced amount of leachate, and in situ leachate treatment. Such advantages have minimized environmental risks, have allowed extension of the useful life of the landfill site, and have fostered cost reduction. LBRs of three types have been developed using both anaerobic and aerobic modes: anaerobic, aerobic, and hybrid. Microorganisms in landfills cause various reactions related with organic fractions and heavy metals. Such functions have been stimulated in LBRs by recirculation of leachate with or without aeration. To date, most studies of microorganisms in LBRs have analyzed bacteria and archaea based on 16S rRNA genes and have analyzed fungi based on 18S rRNA genes from a taxonomical viewpoint. Indicator genes for specific functions in LBRs such as nitrification, denitrification, and methane production have also been monitored. The population dynamics of microorganisms in LBRs have been partially clarified, but the obtained data remain limited because of highly heterogeneous features of solid wastes inside LBRs. Systematic monitoring of microorganisms should be established to improve LBR performance. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.