The view that small prisons are generally more desirable than large ones is presented, and problems concerning the operational definition of the word 'small' are discussed. Two extremes in specifying optimum prison populations are noted: R.D. Barnes, senior architect of the Federal Bureau of Prisons suggests 1,200 inmates as an optimal number; the Canadian Solicitor General's working group concluded that 150 should be the optimum number. Three studies which found prison size to be related to behavior inside or outside the prison are reported; none indicate that prison size is a contributory or causal factor in prison violence. A review of British studies which examined the relationship between juvenile institution size and postrelease behavior found that size does not correlate with reconviction rate. Additionally, English prison statistics are examined, and attention focuses on prison offenses, assaults, and recidivism. These statistics also fail to provide empirical evidence that prison size influences behavior inside or after leaving prison. Reasons for the widespread belief about the undesirability of larger prisons are suggested, and the relationship between prison overcrowding and prison effectiveness is analyzed. Finally, ways to reduce prison overcrowding are suggested. Four tables and 24 references are included.