This chapter focuses on the relationship between writing systems and language, which is never perfect, with the result that irregularities and idiosyncrasies arise even in writing systems that ostensibly have a one-to-one correspondence between grapheme and speech sound (or other unit of language). On the basis of a diverse assortment of examples drawn from around the world, this chapter outlines the ways in which writing systems are and are not systematic and discusses various avenues by which idiosyncrasies arise. The survey begins with a consideration of systematicity at the level of individual graphemes, where both aesthetic and functional aspects are discussed, and follows this with an exploration of the various degrees to which phonetic writing systems cover a language’s phonemic and subphonemic distinctions and where irregularities can arise. Issues of spelling and orthography, already interspersed in the first two parts, are the dedicated topic of the last section. At various points the chapter showcases the tension between desire for economy and efficiency and desire for regularity.