During the last 50 years, the tobacco industry has sought to modify cigarettes with the goal of reducing the risks of smoking. One crucial element of this work has been the development of a greater understanding of smoke chemistry, generally with the aims of identifying which smoke components may be responsible for the human health risks associated with smoking, and which components are important to the flavour and sensory experience of smoking. Views on the relative importance of the various classes of chemicals in smoke for disease generation have changed over the years, as knowledge of smoke has developed. In the 1960s, polynuclear aromatic hydrocarbons were considered to be the most relevant smoke constituents and much work was undertaken to reduce their levels in smoke. By the 1980s it was realised that key cigarette design changes that would reduce polynuclear aromatic hydrocarbons in smoke would also increase nitrosamines, which some now regard as more important in disease generation. This is an example of one of the dilemmas facing those trying to selectively reduce biologically-active chemicals in smoke: reducing one group of substances in smoke usually produces other undesirable effects. Consequently, the most promising approach historically to the development of "less hazardous" cigarettes has been to reduce all of the smoke components rather than selective reduction of specific constituents. This approach has resulted in the gradual reduction of average 'tar' yields in cigarette smoke to less than a third of their levels 30 years ago. Most other smoke constituents have fallen to a similar extent. Some public health scientists have recently raised concerns as to whether the low 'tar' approach is appropriate on the basis that people switching from higher to lower 'tar' cigarettes 'compensate' their smoking behaviour. The large number of studies on human smoking behaviour is also considered briefly and it is concluded that, in general, when smokers switch from a higher to a lower 'tar' yield cigarette, they will obtain a reduction in smoke component uptake.