State-level momentum in support of vouchers and tax credits that help students go to private schools highlights what has been a largely theoretical issue: private school capacity to support voucher-financed enrollment. Academics say the national supply of seats in secular and religious private schools is sufficient to meet short-term demand from existing voucher programs and from those being considered in states such as Pennsylvania. And if the voucher movement continues to gain traction new private schools may be established or old ones expanded, a pattern that took place with the spread of charter schools. Longer term, however, meeting the demand for voucher-funded seats will depend on factors such as the scope of those programs, what grade levels they serve, and whether a program's design encourages private school participation or tangles it in red tape. Private schools have kept up with demand so far, academics say, but voucher programs' design and scale complicate longer-term prospects. The author reports on private schools' capacity issue that looms for vouchers.