The paper under review proves a general \(\Omega\)-theorem for the sum of the coefficients of polynomial combinations of \(L\)-functions from the Selberg class \(\mathcal{S}\) of Dirichlet series with functional equation and Euler product. More precisely, fix a polynomial \(P\in \mathbb{C}[X_1,\ldots, X_N]\) and \(F_1,\ldots, F_N\in \mathcal{S}\), and let \[ H_P(s)=P(F_1(s),\ldots, F_N(s))=\sum^\infty_{n=1}\frac{a_P(n)}{n^s}, \] which may be expressed as a linear combination (if \(H_P(s)\not\equiv 0\)) \[ H_P(s)=\sum^M_{\nu=1}c_\nu G_\nu(s) \] where \(c_\nu\in\mathbb{C}^\times\) and \(G_\nu\in \mathcal{S}\). Let \(d=\max_\nu d_{G_\nu}\), where \(d_F\) is the degree of \(F\in\mathcal{S}\). The first main theorem of the paper states that \[ \sum_{n\leq x}a_P(n)=\Omega(x^{\frac{1}{2}-\frac{1}{2d}})\quad \text{and} \quad \sum_{n\leq x}|a_P(n)|=\Omega(x^{\frac{1}{2}+\frac{1}{2d}}). \] As a direct consequence, the authors provide the second main theorem, which states that the real and imaginary parts of any linear combination of coefficients of such \(L\)-functions have infinitely many sign changes, assuming some simple necessary conditions.