There is now ample evidence of a rise in 'non-standard work arrangements' in many industrialised economies, yet only rarely does theoretical and empirical work probe the question of why the risk of temporary employment varies. Focusing on temporary employment, the author extends the scope of closure theory to an entirely new domain. He develops new and highly sophisticated measures with which to document occupational closure by measuring the scarcity of credentials for particular types of occupations and how effectively these credentials signal quality to potential employers. Similarly, by measuring the degree of task specialisation and the uniqueness of the occupations' tasks, this work exceeds previous studies in breadth and precision. These new measures allow the author to demonstrate how professions protect their employees from temporary employment contracts.