The bright colour of haemoglobin has, from the very beginning, played a significant role in both the investigation of this compound as well as in the study of blood oxygen transport. Numerous optical methods have been developed for measuring haemoglobin concentration, oxygen saturation, and the principal dyshaemoglobins in vitro as well as in vivo. Modern applications include pulse oximeters, fibre optic oximeters, multiwavelength haemoglobin photometers ('co-oximeters') and instruments for near infrared spectroscopy in vivo. Knowledge of the light absorption spectra of the common haemoglobin derivatives is a prerequisite for the development and understanding of these techniques. In the 1960s a reference method based on the absorptivity of a single derivative (haemiglobincyanide; HiCN) at a single wavelength (540 nm) was established for measuring the total haemoglobin concentration. Thus an anchor value was provided on which the absorptivity spectra of all other haemoglobin derivatives could be based. This monograph presents absorption spectra and absorptivity data in the wavelength range of 480 to 1000 nm of the major haemoglobin derivatives for human adult and foetal haemoglobin and for haemoglobin of several animals (cow, dog, horse, pig, rat, and adult and foetal sheep). A detailed description of the methods used to acquire these data has been included to allow future investigators to reproduce and expand on the data. The second part of the monograph includes chapters on the principles and development, in historical perspective, of the principal methods for measuring total haemoglobin concentration for two, three, and multi-component analysis of haemoglobin derivatives, and for blood oxygen saturation measurement. Accurate quantitative data pertaining to haemoglobin in human blood are presented, together with a description of methods for measuring haemoglobin oxygen capacity and oxygen affinity. These chapters have been written with a view to foster knowledge and insight concerning the principles, purposes, and limitations of the various methods of all who use these methods in research and patient care.