We study the space of C 1 isogeometric spline functions defined on trilinearly parameterized multi-patch volumes. Amongst others, we present a general framework for the design of the C 1 isogeometric spline space and of an associated basis, which is based on the two-patch construction [7] , and which works uniformly for any possible multi-patch configuration. The presented method is demonstrated in more detail on the basis of a particular subclass of trilinear multi-patch volumes, namely for the class of trilinearly parameterized multi-patch volumes with exactly one inner edge. For this specific subclass of trivariate multi-patch parameterizations, we further numerically compute the dimension of the resulting C 1 isogeometric spline space and use the constructed C 1 isogeometric basis functions to numerically explore the approximation properties of the C 1 spline space by performing L 2 approximation.