This report describes a simple, efficient, and flexible program for cubic spline interpolation in one dimension, suitable for inclusion in the ANL Subroutine Library and in an interactive timesharing system. Most of the cubic spline programs already available either restrict the boundary conditions to knowledge of the first derivative at both end points, or require the second derivatives to vanish at these points. The program described here removes these restrictions and enables the user to adopt boundary conditions appropriate for his own problem. The advantage is demonstrated for the extreme case of interpolating the function x log (1/x), which has singularities in all its derivatives at x = 0.