Parallel plasma flows with Mach number M≈0.5 have been measured inthe JET scrape-off layer (SOL). These flows show a maximum some 10-20 mmmid-plane outside the separatrix, and decrease towards the separatrix. Theflow reverses when BT and IP are reversed, but is asymmetric aboutzero. A number of possible mechanisms for the observed flow are suggested,including ion Pfirsch-Schlüter flow, ballooning transport, momentumtransfer, reverse flow (ionization driven flow) and co-current toroidalmomentum generated in the SOL by ion ∇ and centrifugaldrifts.