We extend the classical boundary values(0.1)g(a)=−W(ua(λ0,⋅),g)(a)=limx↓ag(x)uˆa(λ0,x),g[1](a)=(pg′)(a)=W(uˆa(λ0,⋅),g)(a)=limx↓ag(x)−g(a)uˆa(λ0,x)ua(λ0,x), for regular Sturm–Liouville operators associated with differential expressions of the type τ=r(x)−1[−(d/dx)p(x)(d/dx)+q(x)] for a.e. x∈(a,b)⊂R, to the case where τ is singular on (a,b)⊆R and the associated minimal operator Tmin is bounded from below. Here ua(λ0,⋅) and uˆa(λ0,⋅) denote suitably normalized principal and nonprincipal solutions of τu=λ0u for appropriate λ0∈R, respectively.Our approach to deriving the analog of (0.1) in the singular context employing principal and nonprincipal solutions of τu=λ0u is closely related to a seminal 1992 paper by Niessen and Zettl [58]. We also recall the well-known fact that the analog of the boundary values in (0.1) characterizes all self-adjoint extensions of Tmin in the singular case in a manner familiar from the regular case.We briefly discuss the singular Weyl–Titchmarsh–Kodaira m-function and finally illustrate the theory in some detail with the examples of the Bessel, Legendre, and Kummer (resp., Laguerre) operators.