To measure something is easy. To make sure that the measurement has real meaning to someone else is much more difficult. There are well-established measurement systems in the physical sciences that have overcome the difficulties but the position in biotechnology and other biosciences is much less satisfactory. The comparability of much of the data obtained in biotechnology - for example from microarrays - is poor; laboratories cannot be certain that their own methods would deliver the same result as that published by others. This is hindering the development of the biosciences and the Measurements for Biotechnology (MfB) programme has been initiated in an attempt to resolve some of the problems. It aims is to engage the bioscience community in developing improved methodology and in discussions about the best way forward. Part of the problem is lack of awareness of the language, practice and underlying principles of metrology, as they have become established in the physical sciences; this primer is an attempt to remedy this. It is written recognising that advances in biology have largely been based on observation accompanied by qualitative measurements, rather than by quantitative ones using metrological standards.