Chapter 18 continues a section of of Leviticus dealing with regulations for the Israelites as it related to holy living. God exhorts Israel not to live like they did in Egypt (where they just came from) or like they lived in Canaan (where they were headed), but to live live separated unto HIm. Specifically, this chapter deals with regulations regarding sexual relationships. In a sex-saturated world, this in extremely timely and relevant.