More than a billion people in today's world lack adequate shelter and live in unsanitary, unsafe conditions which provide little protection from the elements. There is no lack of good ideas and practical solutions for the problem. It is the availability of affordable, durable construction materials that determines how much and what type of construction will take place in a given country. Expenditure on building materials in the Third World absorbs between three and five per cent of GDP and represents an immense drain on foreign exchange. For example, the total Third World import bill for construction material rose from $3.3 billion in 1970 to $35 billion in 1982. Much of the bill was charged to Africa, where in some countries over 90 per cent of the value of commercial building materials is imported. 1 The real challenge, therefore, is to ensure that affordable, durable materials are widely enough produced and in sufficient quantities that they can reach the poor and, where possible, create jobs in the process.