Let (a, b) ⊂ R be a finite or infinite interval, let p 0 (x), q 0 (x), and p 1 (x), x ∈ (a, b), be real-valued measurable functions such that p 0 , p −1 0 , p 2 1 p −1 0 , and q 2 0 p −1 0 are locally Lebesgue integrable (i.e., lie in the space L 1 loc (a, b)), and let w(x), x ∈ (a, b), be an almost everywhere positive function. This paper gives an introduction to the spectral theory of operators generated in the space L 2 w (a, b) by formal expressions of the formwhere 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 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 δ-interactions of intensities h k centered at some points x k , where2010 Mathematics Subject Classification. Primary 34B24; Secondary 34B20, 34B40. Key words and phrases. Second-order quasi-differential operator, minimal operator, maximal operator, Sturm-Liouville theory, deficiency index, limit point-limit circle, linear differential equation with distributional coefficients, finite-difference equation, Jacobi matrix.