From neuron to behaviour, the nervous system operates on many levels of organization, each with its own scales of time and space. Very large sets of data can now be obtained from these multiple levels by the explosive growth of new physiological recording techniques and functional neuroimaging. Among the most difficult tasks are those of conceiving and describing the exchanges between levels, seeing that the scales of time and distance are braided together in a complex web of interactions, and that causal inference is far more ambiguous between than within levels. In this paper, I propose that a generic description of these multi-level interactions can be based on the temporal coordination of neuronal oscillations that operate at multiple frequencies and on different spatial scales. Specifically, the amplitude of the oscillations at each characteristic frequency is modulated by cyclical variations in neuronal excitability induced by lower frequency oscillations and emerging simultaneously on a larger spatial scale. Following this general rule, global patterns of integration can produce downward effects, occasionally acting on and constraining the local level of cell assemblies, whose activity can thus be taken as a signature of the downward influence. This cross-scale framework is firmly rooted in neurophysiology and as such is entirely amenable to experimental testing.