Molecular techniques have become a valuable tool in phytoplankton studies over the past decades. The appropriate choice of a technique from an increasing array of methods can be rather complex, because different techniques are suitable for different questions or problems in ecology and evolution. Each technique has its particular strengths and weaknesses and is based upon different ( theoretical) assumptions. Our aim is to give a better insight in the ( correct) use of various molecular techniques in phytoplankton research, with special emphasis on the fields of strain identification, differentiation of populations and the establishment of phylogenetic relationships. The basic steps in the development of molecular techniques like allozyme electrophoresis, RFLP, DGGE, SSCP, RAPD, AFLP and microsatellites, and the application of these techniques in phytoplankton research, are discussed. Furthermore, recent developments in molecular biology, that have so far only found limited application in phytoplankton studies, such as single-cell PCR, PCR assays combined with molecular probes ( Heteroduplex Mobility Assays or DNA arrays), Real-time PCR, complete genome sequencing, multi-gene expression studies using microarrays, and Single Nucleotide Polymorphism (SNPs), are discussed. We emphasise the relevance of fundamental and applied molecular studies on phytoplankton for a wider community of ecologists and evolutionary biologists.