When Did Literacy Become Widespread?

It partly depends how you define literacy.  Historically many people couldn't even sign their names (except with an X).  Adequate literacy levels, which might be defined as the reading level of a ten year old child with decent educational provision, has only become commonplace and the expected norm since state education became free and widespread.  This started to happen in England in 1870.

Historically literacy rates have often been tied to religious aims.  In Islam, individual reading of the Qu'ran is essential in order to obtain salvation, and thus great emphasis has been placed on learning to read (less consistently for girls than boys, though).  Judaism and Protestantism also place an emphasis on having an individual relationship with God.  An 18th century campaign to make it possible for all Welsh people to read the Bible (in Welsh) gave Wales the highest literacy rates in the world in 1750.

How literate are we now?  Some sources reckon that up to 40% of adults in Britain have sub-optimal literacy skills, with 20-25% having severe problems with reading and writing.