Plant tissue culture provides a powerful technique to assist the plant breeder in improving the propagation and performance of agricultural, horticultural and forest species, to provide the industrialist with a potential source of pharmaceuticals, safe colours and flavours, and to supply the biochemist with standardised plant material for the analysis of plant metabolism. The actmil contribution that tissue culture has made to these fields of endeavour is variable, and is a principal theme of the review chapters of this book, but the ability to distinguish specific phenotypes at the cell level is a recurrent feature of the diverse fundamental and applied investigations in this area. For example, i~w-~_s(jon)IJay lead to the production of plants showing resistl!llCe (or tolerance) to specific environmental stresses, such as low tern erature, salini , drought, herbicides, toxic metals, and pathogens-,- the-pro uctton of resistant cell lines or an ys1s o e res1stan mec m arthe· cellular level and the selection of cell lines for high yields of secondary products such as alkaloids, steroids, terpenes, colourings and flavourings. In addition, contemporary approaches to genetic recombination, such as cell fusion and transformation, are dependent on our ability to identify the desired recombinants.