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We discuss the existence of positive solutions to $-Delta u=lambda f(u)$ in $Omega$, with $u=0$ on the boundary, where $lambda$ is a positive parameter, $Omega$ is a bounded domain with smooth boundary $Delta $ is the Laplacian operator, and $f:(0,infty)o R$ is a continuous function. We first discuss the cases when $f(0)>0$ (positone), $f(0)=0$ and $f(0)<0$ (semipositone). In particular, we will review the existence of non-negative strict subsolutions. Along with these subsolutions and appropriate assumptions on $f(s)$ for $sgg 1$ (which will lead to large supersolutions) we discuss the existence of positive solutions. Finally, we obtain new results on the case of infinite semipositone problems ($lim_{so 0^{+}}f(s)=-infty$).