A program combining the Monte Carlo technique and the Kadanoff block transformation is carried out for the Ising model in two and three dimensions, and the critical point and the critical indices $\ensuremath{\nu}$ and $\ensuremath{\delta}$ are obtained. A sequence of approximations seems to converge to the exact Onsager result in two dimensions, although the convergence of $\ensuremath{\nu}$ is rather slow. In three dimensions, the numbers obtained seem to tend towards the results from exact series expansions.
By using Kadanoff's variational renormalization-group method an Ising model with vacancies is investigated for dimensions up to
d=4 to study the crossover to classical behavior. Up to a crossover dimension $d\ensuremath{\simeq}3.4$, both an optimal critical and tricritical fixed point are located with critical indices that agree well with other data. At $d\ensuremath{\simeq}3.4$, both fixed points merge, representing in this way the crossover to classical behavior. At
d>3.4, indications are found that the two fixed points do not annihilate but form a "marginal" pair that describes both critical and tricritical behavior.