Several naturalists were simultaneously found to be working on the classification system which resulted in several systems being created. The widely accepted system of classification is the five kingdom classification. The classification system is widely based on morphological characteristics, cell structures, nutrition and reproduction modes, body organizations, and evolutionary relationships. The chapter talks about the many natural and artificial classification systems, the history classification and their drawbacks. Amongst this, the kingdom Monera which is prokaryotes is described in details in this chapter.