Spindle oscillations are commonly observed during stage two of non-REM sleep. During sleep spindles, the cerebral cortex and thalamus interact through feedback connections. Both initiation and termination of spindle oscillations are thought to originate in the thalamus, based on thalamic recordings and computational models. Drawing on results obtained with large-scale biophysical network models of the thalamocortical system, I will discuss the role of corticothalamic influences on initiation, termination and synchronization of sleep spindles.