In this course we'll discuss how to use the radionuclides as tracers to study the interactions of natural processes on the Earth. The material to be covered in this class can be broadly put into two sections (semesters). In the first semester we’ll introduce the fundamental theory of radioactivity, including its historical evolution, atom and radiation, radioactive decay and growth, natural and anthropogenic radionuclides, radiometrology etc… In the second semester we’ll be looking at the behaviour of radionuclides in the environment, such as the atmosphere circulation, the ocean circulation and mixing processes, marine bio-geochemistry, sedimentation dynamics, marine pollution, soil erosion, paleo-oceanography and paleo-climatology, environmental radiation monitoring etc…