**1. Introduction**

The relationship between microorganisms and human skin is complex. Some microorganisms are friendly residents, while others are harmful pathogens. Human skin has a variety of mechanisms for interacting with microorganisms, which promote the propagation of certain organisms while attacking others. Each of the different microorganisms discussed in this chapter has a unique relationship with humans. Their stories are not simple but are quite interesting.

To make the stories easier to understand, this chapter is divided into two sections. One section is devoted to the characteristics of the resident microbiota (microflora) of patients with atopic dermatitis (AD), while the other covers secondary infections that frequently occur in AD patients which are sometimes life-threatening. The microbiota of AD patients is significantly different from that of the normal population, and the relationship of microbiota with AD is widely accepted.
