The significance of the Supreme Court's two recent overcrowding cases and the potential legal role of evidence of harmful effects are discussed. Subsequent lower court decisions and the extent to which they consider harmful effects on inmates a decisive factor are examined. Scientific studies of the effects of prison overcrowding on prison rule infractions and violence, mental health, stress and hypertension, and mortality are reviewed. Case studies of trials both before the Supreme Court and lower courts are presented. Results from studies on prison overcrowding indicate that density generally corresponds with the rate of inmate rule violations. The association between density and rule infractions was particularly strong in institutions that housed juveniles and young adults. Within the adult institutions, however, the correlations tended to be low and inconsistent. Evidence does exist that overcrowded prisons increase the rate of inmate illnesses with substantially elevated rates of tuberculosis. A number of studies of inmate mental health have focused on the relationship between overcrowding, stress, and hypertension. Blood pressure examinations were given to 568 inmates from the date of incarceration until the date of release. It was found that the inmates' blood pressure was high during the earliest a stage of confinement, dropped after an initial period of adjustment, and then rose again. Findings also demonstrated a significant change in blood pressure following a change in the level of crowding in the inmate's housing. It is concluded that there is substantial empirical evidence that prison overcrowding is harmful to inmates. It is recommended that plaintiffs should present tangible evidence of the harmful effects of prison overcrowding to support their constitutional challenges; courts should carefully consider such evidence, comparing the circumstances at issue with those described in the empirical studies. Tabular data and 298 footnotes are provided.