The properties and behavior of intertidal marine sediments cannot be understood without taking their biology into account. Biological factors are important for the stability and erosion threshold of intertidal sediments as well as for sediment transport. In this paper I focus on intertidal sediments that are colonized and dominated by phototrophic microorganisms and their impact on the morphodynamics and sediment stabilization. The emphasis is on epipelic diatoms. These organisms exude copious amounts of extracellular polymeric substances (EPS) that may contribute to the stability of the sediment by gluing and binding. I review the factors that lead to the development of such microphytobenthic communities and the processes that lead to the exudation of EPS and its fate in intertidal mudflats. Epipelic diatoms exude EPS partly as the result of unbalanced growth. Extraction of EPS from cultures of epipelic diatoms yields two operational fractions. While one fraction contains largely neutral EPS, which may serve as a carbon- and energy reserve for the organism, the other is acidic and more recalcitrant to degradation. The latter EPS fraction is therefore predominant in the muddy sediment and may be responsible for increasing the erosion threshold. However, since extracted EPS alone is incapable of increasing the erosion threshold, diatoms are apparently actively involved in the structuring of the biofilm matrix. Therefore, sediment stabilization cannot be attributed simply to EPS alone.