Areas of research - Petroleum microbiology: Determination of the microbial diversity of oil reservoirs and oil associated ecosystems (wellhead samples and oil field core samples). This research is performed to investigate if microbes can serve as indicators for changing conditions in subsurface environments and if they have a potential use in so-called ‘smart well applications.’ - Formation of biofilms in subsurface environments: The development of a micromodel (flowcell) in which biofilm formation in porous media environments were visualized and studied on the pore level. Summary: As it is getting more difficult to retrieve oil from the subsurface, there is a renewed interest from the petroleum industry regarding microbial processes in oil-water systems, like oil reservoirs and their associated refineries. Oil fields are specific ecosystems, they are oxygen depleted, contain a variety of hydrocarbons and often have elevated temperatures and pressures. Through human exploitation, active changes in oil field ecosystems are induced. An example is seawater injection to displace oil. Seawater injection causes a decrease in temperature and induces the growth of sulphate reducing bacteria due to the introduction of sulphate and thereby as a consequence the production of harmful H2S. The current idea is that microorganisms detected in, for example, production water from an oil well, hold additional information on the oil field itself and the processes that are occurring in this oil field during exploitation of the field, so-called ‘Biomonitoring’. Through the application of ‘smart well’ technology, viz. clever exploitation of the oil field, more oil can be retrieved from the field. This however requires new information sources from the field itself. Biotechnology might offer an additional information source. Also it is expected that growth of microorganisms in oil field can plug so called ‘thief zones’ in oil fields, which forces injected water to take an alternative route and thereby displacing more of the oil. This process however has first to be understood on the pore level. This thesis investigates the concept of ‘biomonitoring’. To this purpose the microbial community of water and core samples taken from various oil fields, their separation facilities, and other subsurface environments have been investigated with the use of various molecular techniques like denaturing gradient gel electrophoresis (DGGE) and clone library construction of 16S rRNA gene fragments. The presence of several species can be confirmed when looking at their genomic material. Presence of specific species is correlated to the characteristics of the environments. It was shown that this is indeed possible. Also progress has been made in the research towards biofilm development in oil field rocks. To this purpose a glass (2D) micromodel (flowcell) has been developed. This micromodel contained idealized oil field rock structures. Biofilm formation has been induced in this micromodel and it was shown that biofilms can indeed plug pores.