Interactive proof systems in which the Prover is restricted to have a polynomial size strategy are investigated. The restriction of polynomial size computation tree, or logarithmically bounded number of coin flips, guarantee a polynomial size strategy. One result is that interactive proof systems in which the Prover is restricted to a polynomial size strategy are equivalent to MA, Merlin-Arthur games, of Babai and Moran [l]. Polynomial tree size is also equivalent to MA, but when logarithmic space is added as a restriction, the power of polynomial tree size reduces to NP. A logarithmic number of coin flips is equivalent to NP, even when logarithmic space is added as a restriction.