Abstract. In this paper, we study bounded solutions of −∆u = f (u) on R n (where n = 2 and sometimes n = 3) and show that, for most f 's, the weakly stable and finite Morse index solutions are quite simple. We then use this to obtain a very good understanding of the stable and bounded Morse index solutions of − 2 ∆u = f (u) on Ω with Dirichlet or Neumann boundary conditions for small .The purpose of the present paper is to study the equationon R n (or a half space T ), where we are interested in solutions which are weakly stable or have finite Morse index (that is, have only finitely many negative eigenvalues in some suitable generalized sense). Usually, we study positive solutions (but not always). For n = 2 and sometimes for n = 3 we prove that these solutions are very simple and easy to understand. (For example, if n = 2, the weakly stable solutions usually have to be constant.) This is an interesting contrast to the results in [8] where for certain nonlinearities there are a great many positive bounded solutions which are periodic in some variables and decay in others. In addition, for many f , it is easy to use variational methods (or bifurcation methods) to construct many solutions periodic (and non-constant) in all variables. Thus it seems that the structure of all solutions of (1) may be quite complicated. Our results show that the finite Morse index solutions are usually quite simple. We also prove closely related results on half spaces.As an application of these ideas, we have a number of results on the solutions (usually but not always positive) ofwhere Ω is a bounded open set in R 2 (sometimes in R 3 ) and we have homogeneous Neumann or Dirichlet boundary conditions. For many f 's we obtain the exact number of stable positive solutions for small positive and show it is independent of the shape of the domain Ω (and easy to calculate). This contrasts strongly with the results for all positive solutions in [9], [10] and [11] for small positive and results for the stable positive solutions when is not small. We also prove that stable positive solutions are constant in the case of Neumann boundary conditions, and for either boundary condition there are no stable sign-changing positive solutions for small positive (for many f 's). These two results contrast with results in [31] and [16].