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The Blue Book, formally titled Disability Evaluation Under Social Security, lists impairments the Social Security Administration (SSA) considers severe enough to prevent someone from working and lays out the medical criteria for determining if that person can receive disability benefits.

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Formerly printed and bound but now published solely online, the Blue Book is a critical tool for Social Security examiners who weigh applications for Social Security Disability Insurance (SSDI) and Supplemental Security Income (SSI), the two disability benefit programs the SSA administers, and for medical professionals who furnish evidence to support patients’ disability claims.
If a condition is in the Blue Book, it inherently meets SSA's definition of disability: An illness or injury that prevents you from performing "substantial gainful activity" (Social Security–speak for most paying work) for at least a year or that will likely result in your death. For children, who can receive SSI, the test is not work-related but rather whether a condition causes “marked and severe functional limitations."
Simply being diagnosed with a listed condition does not automatically qualify you for SSDI or SSI. The Blue Book spells out in detail the symptoms, test results or other data that show your condition is acute enough to be genuinely disabling in the agency's eyes and the records you must produce to prove it.
As part of its disability-determination process, SSA examiners review your evidence against the Blue Book listing to see if you meet the requirements. If you do, that's usually sufficient for your claim to be approved. But it isn't necessarily disqualifying if you don't. (See “Keep in mind" below.)