"Good cause" for remand due to Secretary's failure to develop record. ALJ is not at liberty to substitute his own impression of an individual's health for uncontroverted medical opinion. Given unchallenged diagnosis of chronic schizophrenia, coupled with uncontradicted testimony concerning claimant's degeneration in recent years and evidence of gross interference with his interpersonal relations, finding of no severe impairment was not supported by substantial evidence. If Secretary is doubtful as to severity of claimant's mental disorder, appropriate course is to request a consultative evaluation, not to rely on the lay impressions of the ALJ. ALJ incorrectly applied "Listing" standard at Step 2. Certain mental abilities are a prerequisite to working: the regulations suggest as prerequisite to working: the ability to understand, carry out and remember simple instructions, to use sound judgment, to respond appropriately to supervision, co-workers and usual work situations and to deal with changes in a routine work setting. While claimant bears the burden of proof on the issue of disability, the Secretary nonetheless retains a certain obligation to develop an adequate record from which a reasonable conclusion can be drawn.

A doctor's conclusory reflections as to claimant's employability, unaccompanied by any formal opinion and diagnoses, do not constitute substantial evidence. Due to non-adversarial nature of proceedings, SSA's responsibility to develop the evidence increases where claimant is unrepresented, where the claim seems on its face to be substantial, where there are gaps in the evidence needed for a reasoned evaluation of the claim, and where it is in the power of the ALJ, without undue effort, to see that the gaps are somewhat filled.

Neither absence of treatment records from relevant time period and retrospective nature of treating doctor's opinion justify ALJ's finding that treating source's report was too speculative a basis for establishing a severe impairment. (remanding for ALJ to obtain retrospective opinions regarding claimant's mental condition in relevant period). Because of plaintiff's readily apparent mental disorder and fact that the ALJ noticed the possibility that the disorder might be related to an event occurring prior to the date last insured., ALJ had duty to develop the record of the etiology of the illness, its course, and its severity. Fairness dictates that when a claimant obviously suffering from a severe mental disorder appears at a social security proceeding without counsel, the ALJ undertake to protect his interests at the hearing. ALJ's failure to develop evidence is good cause for remand. Claimant had burden of establishing by credible evidence that his mental impairment was of a disabling level of severity as of date last insured. It is not sufficient for him to establish that his mental impairment had its roots prior to that date.

ALJ has duty to develop evidence of side effects of seizure medication where claimant alleged side effects disabled him from working. Where claimant asserted that seizure medication made him so sleepy, hot and ill-tempered so as to disable him from working, ALJ should have sought further medical evidence or made further inquiry as to whether side effects of medication was disabling.

At Step 4, the ALJ is entitled to credit the claimant's own description of her past work and her functional limitations, but the ALJ has some burden independently to develop the record. Citing Santiago, 944 F.2d 1 (1st Cir. 1991). Remand. Illegibility of non-trivial parts of medical record, combined with identifiable diagnoses and symptoms indicating more than a mild impairment, alerted ALJ to need for expert guidance regarding the extent of the claimant's RFC to perform her past employment. Case remanded. With a few exceptions, an ALJ, as a lay person, is not qualified to interpret raw data in a medical record. Where claimant sufficiently put her inability to perform her past work in issue, the ALJ must measure the claimant's capabilities, and ordinarily an expert's RFC evaluation is essential unless the extent of functional loss, and its effect on job performance, would be apparent even to a lay person. Ventricular tachycardia, frequent PVCs, sinus tachycardia, arterial hypertension

At Step 4, claimant will be found not disabled when she retains the RFC to perform the actual functional demands and job duties of a particular past relevant job, as she performed it. The claimant is the primary source for vocational documentation. It's claimant's duty to describe her impairments so as to raise the point to the Commissioner how the impairments preclude the performance of her prior work. Where the testimony and evidence did not go far enough to raise a "meaningful issue" in regard to how claimant's impairments affected her ability to work, the ALJ had no duty to further develop the record by obtaining RFC assessments.

Discussion of observable signs of pain. ALJ under no duty to go to "inordinate lengths to develop a claimant's case." Holding that Secretary is not at the mercy of every claimant's subjective assertions of pain applies equally well to subjective claims of dizziness. Claimant must show that her impairment is "medically determinable" and only in a rare case can she do this without medical evidence.