This article reviews the basic principles of immunization, identifies the components of the practice of immuniz.ation, and points to some of the issues specific to that practice that will need to be taken into account as the vaccines of the future are coming close to availability. The purpose of immunization is to protect an individual from a specified infectious disease, from the earliest appropriate age, for as long as possible, using the fewest number of doses to achieve that immunity, and

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... th the least possible risk from the procedure. For certain diseases, for example tetanus, the benefit of immunization is only to the vaccine recipient. In the case of vaccines such as polio vaccine, there is a wider purpose. As well as protecting immunized individuals, there can be community benefit to individuals not immunized. When sufficiently high coverage is reached, transmission is interrupted and individuals not immunized are further protected. For routine immunization against any vaccine-preventable disease, there needs to be the provision of routinely available processes that seek to promote the highest possible coverage in the target population; allow for the measurement of that achievement in an accurate and timely way; detect any possible adverse effects of the immunization; and sensitively and rapidly provide information on the target diseases. As the availability of existing resources for health programs comes under increasing scrutiny, countries in all stages in development will need to consider the most co'st-effective use of resources, especially as countries are encouraged to become self-sufficient for financing their immunization programs. Finding the necessary resources for presenlt vaccines, let alone the vaccines of the future, may be a considerable challenge.