What do you know about how the doctor gets this picture for diagnosis? Have you ever asked how the sun produces light and heat? What science enbled man to explode the atomic bomb about which you so often hear? How do they diagnose ailments due to malfunctioning of thyroid glands using radioactive tracers? How is power generated in Kalpakkam nuclear plant? The underlying science presupposes knowledge of Radioactivity. This section presents just one aspect of this science known as Nuclear Science that has an impact on our lives since Becquerel discovered radioactivity, viz., how do we detect and measure radioactivity.