The meaning of many natural language expressions depends systematically on context: Cases like 'I', 'here', and 'now' are most frequently cited, but there are many others. Accounting for the interrelationship of context and content emerged in the first half of the twentieth century as a surprisingly perplexing challenge for theorists. Workable solutions to the problem were slow to emerge, but by 1970 an approach had appeared that could be adapted to the purposes of formal semantics. As usual, these developments left a residue of unresolved philosophical problems.