A slug barrier comprises a support 12 for installation at a boundary of a controlled zone； and an array of spikes 24, 26, 28 extending from the support, the array comprising spikes of at least first and second differing lengths arranged such that spikes of the first length are interposed between spikes of the second length. The barrier may include a plurality of rows of spikes for intersecting a slug path in use and a row comprising spikes of the first length is interposed between rows comprising spikes of the second length. The rows may be offset relative to neighbouring rows. The rows of spikes may comprise rows of a third length alternating with spikes of the first or second length, the spikes of the third row being difference from the first and second lengths.
