At least 10 million kinds of organisms inhabit the Earth but less than one third of these have been studied and catalogued (put in record) by biologists. Diversity among the Earth’s organisms is more obvious than the fundamental unity of life. We see that all organisms share many biological characteristics. Five principal groups of organisms are prokaryotes, protists, fungi, plants, and animals. In this chapter we will focus on the differences among different groups of organisms. We will also see how are organisms classified and named and what are the concerns with the existence of biodiversity.