The effects of several microbial, chemical and physical parameters on bacterial attachment was studied in batch adsorption and column transport studies. Specifically, the role of bacterial characteristics, ionic strength of solution, roughness of solid surfaces, and hydraulic conductivity were tested. Using particle removal theory, the collision efficiency factor (α) was calculated from the breakthrough of the cells (C/Co) in the columns. The values of α varied over three orders of magnitude. The distance that the cells were predicted to travel through the porous medium was calculated assuming C/Co = 0.05 (95% retention). Predicted travel ranged from less than 5 cm to almost 10 km. Bacterial surface characteristics that apparently affected attachment were electrophoretic mobility, hydrophobicity, and biopolymer production. Electrostatic interactions, described by DLVO theory, governed attachment of hydrophilic cells to hydrophilic surfaces. Hydrophobic interactions dominated attachment of hydrophobic cells. Steric hindrance may have resulted from the adsorption of extracellular biosurfactant onto cell and solid surfaces. Size also influenced retention of the cells in the porous medium. Cells less than 0.1 μm and greater than 3.0 μm in diameter were retained to a greater extent than cells with diameters within this range. Increased ionic strength resulted in increased attachment of cells. Attachment to sand grains was greater than attachment to glass beads, possibly because of the rougher surfaces. Lower hydraulic conductivity resulted in greater retention of cells.