Genetically modified animals were created about 30 years ago, and are considered good models of human diseases. In this chapter, the types of genetically engineered mice, such as transgenic, knockout conditional knockout, and knockin animals, and the general techniques on how to obtain them are described. In addition, the available genetically modified models for genetic diseases, multifactorial and polygenic (complex) disorders, neurodegenerative, inflammatory diseases, besides cancer models, are presented. The aim of the chapter is to help investigators to find out the best models for their studies.