Investigation of travertine accumulations throughout central Italy and the west-central U.S. has shown that the carbonate is precipitated in response to both inorganic and organic processes. Individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent; all of the carbonate is low-magnesian calcite. Water chemistry, temperature, and morphology of the accumulation greatly influence the constituents comprising these deposits. Harsh environmental situations favor inorganic deposits, while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material. Morphological variations of travertine deposits recognized include 1) waterfall or cascade, 2) lake-fill, 3) sloping mound, fan, or cone, 4) terraced mound, and 5) fissure ridge. Gross morphology, internal stratification, and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters. Bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations, exceeding 90% of the framework grains comprising some of the lake-fill deposits. Bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents. The bacteria are primarily rods, generally 0.2 mu m in diameter and less than 1.0 mu m in length. The rods readily decay resulting in calcite crystals loaded with micropores. The basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 mu m in diameter enclosed in a single crystal of calcite. Aggregates of these crystals produce a variety of deposits including 1) crudely laminated carbonate mud, 2) finely laminated layers of mud, 3) intraclasts, 4) foam rock, and 5) shrubs. The shrubs are most striking, commonly forming layers 1-3 cm thick but also producing bacterial pisoids. At some locales, bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit. The shrub layers are the result of flourishing summer growth of bacteria and, furthermore, show remarkable daily laminae 0.1-0.5 mm thick. The importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits.