Tanzania is a major source of globally significant biodiversity, ranking among the top countries in tropical Africa in terms of the number of distinct ecoregions represented, and in species richness/species endemism. This chapter presents an overview of biodiversity of Tanzania with special emphasis to species numbers and status in the country. It also explains the trends and threats facing biodiversity. In addition, the chapter describes the physical features as well as major ecosystems of the country. Tanzania harbors different physical features including: High Mountains and highlands where northern and southern highlands are found, lowland such as Kilombero valley and all along the coastline and moderate plain land mostly occurring in the central part of the country. The ecosystems in Tanzania are broadly divided into two main categories, namely: terrestrial and aquatic ecosystems. Terrestrial ecosystems include forests, mountains, dry lands, savannah and agricultural lands. Aquatic ecosystem is comprised of two categories including marine and freshwater ecosystems.