Among fatal and non-fatal casualties in fires in buildings, the annual proportion due to smoke and toxic gas has increased threefold over a period of seventeen years; over half the total fatalities in fires are now due to this cause. This paper briefly reviews the problem of smoke and toxic gases in fires and describes investigations aimed at determining whether the introduction of non-traditional materials, principally plastics, into buildings is likely to make a significant increase in the numbers of fire casualties overcome by gas or smoke. The investigations covered the pyrolysis of plastics under laboratory conditions, large-scale fire tests in which the generation and survival of significant toxic products were monitored, and measurements of the toxicity of fire gases. Results are given for rigid PVC used as wall linings and for flexible polyurethane foam burning in bulk. It is clear that large industrial fire loads of these materials can produce an unacceptable hazard, but further work is needed before conclusions can be reached for fire loads relevant to, for example, private dwellings.