This chapter gives evidence of the contemporary controversies, and suggests the variety of roles that parliaments are expected to play in modern political systems. It indicates the widespread existence of the institution, the vastly different forms it takes, and some of the reasons for its remarkable durability. Although parliament can be defined structurally, it cannot be identified by the particular functions it performs in political systems, for these have varied too widely to have any common denominator. Parliament was an old, established political institution before the advent of those characteristics of political systems which we regard as the marks of modernity. Democracy created the first modern crisis for parliament. With the expansion of the suffrage came political parties to mobilize the new electorate, to recruit representatives from it, and to control these representatives after their election.