We introduce the explosion problem for semilinear elliptic and parabolic PDEs. The explosion problem is a question of existence of nonnegative solutions to −Lφ = λg(x, φ) and nonnegative global solutions to ∂tφ−Lφ = λg(x, t, φ). Under certain conditions there is a λ∗ > 0, called the explosion threshold, for which λ < λ∗ implies a solution exists, and λ > λ∗ implies no solution exists. In this p...