[en] We introduce and describe general criteria which characterize long-pulse neutron sources, with a view to guiding and facilitating subsequent instrument design and optimization for specific applications. The ensuing analysis shows that a long-pulse neutron source allows for the possibility of a wide range of flexible instrument concepts with variable resolution and dynamic range, tasks which invariably require the implementation of pulse-modulation techniques in the time domain, particularly for high-resolution applications. We also consider in some detail yet-to-be-tapped opportunities in the use of shorter proton pulses, characterised by a duration commensurate with typical moderation times at spallation sources. (paper)