Requirements for a Security Clearance 

1) If you are a CT, IS, IT, or in another clearance requiring field you will begin completing your Single Scope Background Investigation (SF 86 – Top Secret and SF 85 – Secret) once you begin your basic enlisted or officer training. 

2) The SSBI can be a laborious process. It is a thorough investigation of your life going back 10 years and then every 5 year it is repeated once you are granted a clearance. Here are the actual forms, so be prepared to supply all of this information. 30 days are allowed, during which data must be input. Having the older paper form can be helpful for collecting and organizing the information in advance. The full investigation can take upwards of a year in some cases. An SSBI involves agents contacting employers, coworkers and other individuals. Standard elements include checks of employment; education; organization affiliations; local agencies; where the subject has lived, worked, or gone to school; and interviews with persons who know the individual. The investigation may include an NACLC on the candidate’s spouse or cohabitant and any immediate family members who are U.S. citizens other than by birth or who are not U.S. citizens. 

http://www.opm.gov/e-qip/). 
http://www.opm.gov/Forms/pdf_fill/sf86.pdf

3) Polygraph: Some career fields will also require you to take a polygraph. The most common examinations are Counter Intelligence (CI) and Full-Scope (Lifestyle) polygraphs. While a positive SSBI is sufficient for access to much of the CT/Intelligence community, polygraphs are routinely administered for access to particular agencies which are likely to be assigned to throughout your career (NSA, NGA, DIA, etc.) 

CI Poly: Questions about your truthfulness, decision making, and ability to put national security first. 

Lifestyle: Very thorough, almost invasive, questioning about your life, to include: drugs, alcohol, mental stability, sexual orientation, debt, political inclinations, and any other factor that could be exploited by individuals wishing to bring harm to the U.S. 

4) Interim Clearances: At lower levels, interim clearances may be issued to individuals who are presently under investigation, but who have passed some preliminary, automatic process. Such automatic processes include things such as credit checks, felony checks, and so on. An interim clearance may be denied (although the final clearance may still be granted) for having a large amount of debt, for having admitted to seeing a doctor for a mental health condition, or for having admitted to other items of security concern (such as a criminal record or a history of drug use or alcohol abuse.) 

5) Foreign Ties: Ties to foreign associates, non-citizen family members, or dual citizenship will also raise red flags, but will not necessarily prohibit you from getting a clearance. But you must be a U.S. citizen and renounce your dual status. Expect your time line to take much longer than the average person. 
Why: foreign influence and foreign preference. Dual citizenship in itself is not the major problem in obtaining or retaining security clearance in the United States. If a security clearance applicant's dual citizenship is "based solely on parents' citizenship or birth in a foreign country", that can be a mitigating condition. However, exercising (taking advantage of the entitlements of) a non-U.S. citizenship can cause problems. For example, possession and/or use of a foreign passport is a condition disqualifying from security clearance and "... is not mitigated by reasons of personal convenience, safety, requirements of foreign law, or the identity of the foreign country" as is explicitly clarified in a Department of Defense policy memorandum which defines a guideline requiring that "... any clearance be denied or revoked unless the applicant surrenders the foreign passport or obtains official permission for its use from the appropriate agency of the United States Government". This guideline has been followed in administrative rulings by the United States Department of Defense (DoD) Defense Office of Hearings and Appeals (DOHA) office of Industrial Security Clearance Review (ISCR), which decides cases involving security clearances for Contractor personnel doing classified work for all DoD components. In one such case, an administrative judge ruled that it is not clearly consistent with U.S. national interest to grant a request for a security clearance to an applicant who was a dual national of the U.S. and Ireland.

6) Minimum investigative scope: Past ten (10) years or to age 18, whichever is less. 

7) Expansion of Investigation: The investigation may be expanded as necessary, to resolve issues and/or address employment standards unique to individual agencies. 

8) National Agency Check: Checks on subject and spouse/cohabitant of investigative and criminal history files of the Federal Bureau of Investigation, including submission of fingerprint records on the subject, and such other national agencies (DCII, INS, OPM, CIA, etc.) as appropriate to the individual's background. 

9) Subject Interview: Required in all cases and shall be conducted by trained security, investigative, or counterintelligence personnel to ensure full investigative coverage. An additional personal interview shall be conducted when necessary to resolve any significant information and/or inconsistencies developed during the investigation. In departments or agencies with policies sanctioning the use of the polygraph for personnel security purposes, the personal interview may include a polygraph examination, conducted by a qualified polygraph examiner. 

10) Birth: Independent certification of date and place of birth received directly from appropriate registration authority. 

11) Citizenship: Subject must be a U.S. citizen. Independent verification of citizenship received directly from appropriate registration authority. For foreign-born immediate family members, verification of citizenship or legal status is also required. Immediate family -- spouse, parents, brothers, sisters, children, and cohabitant of the individual requiring access are tested. 

12) Education: Independent verification of most recent or most significant claimed attendance and/or degree/diploma within the scope of investigation. If all education is outside of the investigative scope, the last education above high school level will be verified. 

13) Employment: Direct verification through records of all periods of employment within scope but in any event the most recent two (2) years. Personal interviews of two sources (supervisor/coworkers) for each employment of six months or more shall be attempted. In the event that no employment exceeds six months, interviews of supervisor/coworkers shall be attempted. All periods of unemployment in excess of sixty (60) days shall be verified through records and/or sources. All prior federal/military service and type of discharges shall be verified. 

14) References: Four required (at least two of which are developed). To the extent practical, all should have social knowledge of subject and collectively span the entire scope of the investigation. As appropriate, additional interviews may include cohabitant(s), ex-spouses, and relative(s). Interviews with psychological/medical personnel are to be accomplished as required to resolve issues. 

15) Neighborhood: Interviews with neighbors for last three years if residence exceeds six months. Confirmation of current residence shall be accomplished regardless of length, including review of rental records if necessary. In the event no residence exceeds six months, interview of neighbors should be undertaken.

16) Credit: Verification of the subject's financial status and credit habits of all locations where subject has resided, been employed, or attended school for six months or more for the last seven (7) years. 

17) Local Agency Checks: A check of appropriate Police records covering all locations where subject has resided, been employed, or attended school for six months or more during the scope of investigation, including current residence regardless of duration. In the event that no residence, employment, or education exceeds six months, local agency checks should be performed. 

18) Public Records: Verification of divorce(s), bankruptcy, etc., and any other court (civil or criminal) actions to which subject has been or is a party within the scope of investigation, when known or developed. 

19) Transferability: Investigations satisfying the scope and standards specified above are transferable between agencies and shall be deemed to meet the investigative standards for access to collateral Top Secret/National Security Information and Sensitive Compartmented Information. No further investigation or reinvestigation prior to revalidation every five years will be undertaken unless the agency has substantial information indicating that the transferring individual may not satisfy eligibility standards for clearance or the agency head determines in writing that to accept the investigation would not be in the national security interest of the United States. 

20) Maintaining Your Clearance: Many of incidents can cause your access to classified information to come under scrutiny. These include, but are not limited to: sleeping walking, talking in your sleep, close personal contact with non-status immigrant and/or foreign associates, debt exceeding 30% of your income ratio, HIV/AIDS, prostitution, drug and/or excessive alcohol use, violent offenses, DUI/DWI, infidelity, theft, fraud, external employment simultaneous to your cleared job, marrying foreign nationals, not passing a polygraph, failure to report breaches of access, failure to report other incidents or change in your personal, maritial, or living status to your security investigation office that are related to any of the themes above, mental instability, suicidal thoughts, and many more. 

References 

The Standard Form 86 (SF86) is required to begin the background check process. See the "Questionnaire for National Security Positions" at http://www.opm.gov/Forms/pdf_fill/sf86.pdf 

Myth: You can outsmart a polygraph. In addition to the machine the polygrapher is a trained behavioralist who can read your every move and expression. The physiological impact (stress, respiration, etc) of lying are not avoidable. The shear attempt to try and lie will highlight your lack of integrity. Individuals without a distinguishable physiological response between lies and truths are noted as ‘failure to pass’ as well. It is very important to the U.S. Government to be able to, beyond a shadow of a doubt, the integrity of the individuals ensuring our national security.