2 Answers

Government classified information is governed by the Treasury Board Policy on Government Security, the Security of Information Act and Privacy Act. Only those that are deemed to be loyal and reliable, and have been cleared are allowed to access sensitive information.
Checks include basic demographic and criminal record checks for all levels, and, depending on an individual appointment's requirements, credit checks, loyalty, and field checks might be conducted by the RCMP and/or CSIS.
Clearance is granted, depending on types of appointment, by individual Federal government departments or agencies or by private company security officers. Those who have contracts with Public Works and Government Services Canada are bound by the Industrial Security Program, a sub-set of the GSP.
To access designated information, one must have at least reliability status (see Hierarchy below). Reliability checks and assessments are conditions of employment under the Public Service Employment Act, and, thus, all Government of Canada employees have at least reliability status screening completed prior to their appointment.[1] However, Government employees by Order-in-council are not subjected to this policy.[2]
Clearances at the reliability status, confidential, and secret levels are valid for 10 years, whereas top secret is valid for 5 years. However, departments are free to request their employees to undergo security screening any time for cause.[3] Because security clearances are granted by individual departments instead of one central government agency, clearances are inactivated at the end of appointment or when an individual transfers out of the department. The individual concerned can then apply to reactivate and transfer the security clearance to his/her new position.[1] 

Read more > http://en.wikipedia.org/wiki/Security_clearance 

For what position?