The paper presents an analysis of multi-peak solutions of the following singularly perturbed problem \[ \begin{cases} \varepsilon^2\Delta u- u+ f(u)=0\quad &\text{in }\Omega,\\ u> 0\text{ in }\Omega,\;u=0\quad &\text{on }\partial\Omega,\end{cases} \] where \(\Omega\) is a smooth domain in \(\mathbb{R}^N\) (\(\Omega\) does not have to be bounded) and \(\varepsilon\) is small parameter; the term \(f(u)\) is a superlinear, subcritical nonlinearity. The analysis is based on a variational method. By modifying the nonlinearity and adding a penalization term the authors introduce a new penalized energy functional and analyze its critical points. Section 1 of the paper includes the analysis of a single peak case and Section 2 treats the general multi-peak case.