In 2007 we celebrate the 300th anniversary of the birth of Linnaeus, the eighteenth century Swedish physician best known for developing a pragmatic approach to naming the basic units of biological diversity. Linnaeus leaned heavily on the work of earlier scientists, but he was relentless in applying his new binomial approach across the whole of biology, and to material brought back from all over the world. This great work of synthesis, and its near comprehensive systematisation of the diversity of life as it was known at the time, established Linnaeus as one of the great scientists of his day. It also set new standards and was of great practical value. It became an essential tool in organising the explosion of knowledge that resulted from rapid exploration of the natural world through the late eighteenth and nineteenth centuries. In this sense, Linnaeus made possible the insights of Darwin and Wallace. Linnaeus’ approach to recognizing and circumscribing the basic units of diversity, which he then named, was comparative. It was also based on examination of reference specimens. But it was nevertheless largely intuitive and resulted in a concept of species identity that was fixed. It is remarkable that despite current knowledge of how species develop through the processes of evolution, despite ever deeper insights gleaned from genetics, and despite advances in the theory of systematics, the approach employed by Linnaeus and other naturalists of the eighteenth and nineteenth centuries persists and remains central to much of our modern understanding of biological diversity. In part this is because their work has stood the test of time: we now know that many of the species that they recognized do have a meaningful biological identity. But the seemingly sparse application of more sophisticated techniques to circumscribing species in Nature also undoubtedly reflects the daunting practicalities: outside the most conspicuous groups of animals the variety of life is vast, poorly sampled and very imperfectly understood. For the specialist the unevenness in our current concepts of species identity represents a challenge, and fertile ground for developing deeper understanding. But in other areas such unevenness is inconvenient. For example, we depend on reliable notions of species identity to help set policy for the conservation and sustainable use of natural resources. Instability can be uncomfortable and can have political and economic ramifications. Legislation designed to regulate the conservation and use of species, for example the Convention on International Trade in Endangered Species, or similar efforts at the national level, all rest on the presumption that the relevant species can be reliably and unambiguously identified. Beyond these practical issues the fundamental nature of species remains controversial. Many of these lively arguments revolve around matters relating to different notions of species identity, including how species recognize each other, and how they are recognized by us. Another controversy is over the rules by which species are named: an area of debate confounded by an unfortunate weakness inherent in Linnaean binomial nomenclature. Uniform application of the binomial approach requires uniform concepts of both genus and species identity. The binomial system, rather unfortunately, intertwines the delivery of straightforward species labels, with the much trickier issue of how similarities and differences translate into hypotheses of evolutionary relatedness at the generic level. The focus of this lecture will be on concepts of species identity in plants. The examples will illustrate the heterogeneous state of current knowledge. Many economically important or biologically interesting plants have been well studied and provide insights into the identity of some plant species in genetic and evolutionary terms. But at the other end of the spectrum many plant species (perhaps the majority) are not well understood. Their identity rests on a preliminary interpretation of similarities and differences in plant form, that remain to be understood in terms of the biological processes that created them.