We consider various lattice models of polymers: lattice trees, lattice animals, and self-avoiding walks. The polymer interacts with a surface (hyperplane), receiving a unit energy reward for each site in the surface. There is an adsorption transition of the polymer at a critical value of $β$, the inverse temperature. We present a new proof of the result of Hammersley, Torrie, and Whittington (1982) that the transition occurs at a strictly positive value of $β$ when the surface is impenetrable, i.e. when the polymer is restricted to a half-space. In contrast, for a penetrable surface, it is an open problem to prove that the transition occurs at $β=0$ (i.e., infinite temperature). We reduce this problem to showing that the fraction of N-site polymers whose span is less than $N/\log^2 N$ is not too small.