In this review we report on the culturable and nonculturable microbial communities present in caves. A high diversity was found using both methods; however, identification methods including PCR amplification of 16S rRNA genes (16S rDNA) and community fingerprinting by denaturing gradient gel electrophoresis (DGGE) resulted in a greater bacterial taxonomic diversity, and allowed the detection of unexpected and unknown bacteria. Growth of cave bacteria at different temperatures was investigated. The data indicate that most cave bacteria can grow at temperatures in the range of 5 to 45[degree]C. However, a higher diversity was obtained when incubation was carried out at 13[degree], although incubation at this temperature drastically reduced the number of sporoactinomycete isolates. At 13[degree]C the bacteria oxidized on average 87% of carbon sources provided, while incubation at 28[degree]C reduced this to about 50% carbon source utilization. Selected actinobacteria isolates were tested for the formation of crystals. Strains from all tested genera except isolates of Gordonia and Nocardia produced vaterite and/or calcite. Production of Mg-calcite was restricted to strains of Brachybacterium, Paenibacillus, Rhodococcus, and Streptomyces, while struvite was only precipitated by an unidentified isolate. These findings indicate that actinomycetes may play a role in the formation of mineral deposits in caves.