Let $ (a,b)\subset \mathbb{R}$ be a finite or infinite interval, let $ p_0(x)$, $ q_0(x)$, and $ p_1(x)$, $ x\in (a,b)$, be real-valued measurable functions such that $ p_0,p^{-1}_0$, $ p^2_1p^{-1}_0$, and $ q^2_0p^{-1}_0$ are locally Lebesgue integrable (i.e., lie in the space $ L^1_{\operatorname {loc}}(a,b)$), and let $ w(x)$, $ x\in (a,b)$, be an almost everywhere positive function. This paper gives an introduction to the spectral theory of operators generated in the space $ \mathcal {L}^2_w(a,b)$ by formal expressions of the form
$$
l[f]:=w^{-1}\{-(p_0f')'+i[(q_0f)'+q_0f']+p'_1f\},
$$
where all derivatives are understood in the sense of distributions. The construction described in the paper permits one to give a sound definition of the minimal operator $ L_0$ generated by the expression $ l[f]$ in
$\mathcal {L}^2_w(a,b)$ and include $ L_0$ in the class of operators generated by symmetric (formally self-adjoint) second-order quasi-differential expressions with locally integrable coefficients. In what follows, we refer to these operators as Sturm–Liouville operators. Thus, the well-developed spectral theory of second-order quasi-differential operators is used to study Sturm–Liouville operators with distributional coefficients. The main aim of the paper is to construct a Titchmarsh–Weyl theory for these operators. Here the problem on the deficiency indices of $ L_0$, i.e., on the conditions on $ p_0$, $ q_0$, and $ p_1$ under which Weyl's limit point or limit circle case is realized, is a key problem. We verify the efficiency of our results for the example of a Hamiltonian with $ \delta $-interactions of intensities $ h_k$ centered at some points $ x_k$, where
$$
l[f]=-f''+\sum _{j}h_j\delta (x-x_j)f.
$$
