Starting from this chapter, we discuss string theory for particle physics. Our emphasis will be, starting from string theory, understanding low-energy physics described by the Standard Model (SM). As summarized in Chap. 2, the SM is a chiral theory for fermions, and hence obtaining a chiral spectrum from string theory is of utmost importance. This goes with string theory with fermions, i.e., superstring rather than bosonic string. Superstring is written in ten spacetime dimensions(10D) but the effective quantum field theory of the SM is in four spacetime dimensions(4D), and the extra six dimensions(6D) must be cleverly hidden from the 4D observers. For hiding these extra dimensions, we follow the compactification scheme via orbifolds introduced in Chap. 3 and applied to quantum field theory in Chap. 5. It will be exploited fully in string theory in the subsequent chapters. In this chapter, we introduce basics in string theory.