Pula airport at the southern tip of the peninsula is the easiest way in. Direct seasonal flights connect from London, Manchester, Frankfurt, and several other European cities. Out of season, you usually fly to Zagreb or Trieste and drive from there.
By car, Istria connects directly to Slovenia and Italy: Trieste is about 90 minutes from Pula, Ljubljana about two hours, Venice four. The Učka tunnel under the mountains opens the route to Zagreb and Rijeka. There is no rail line into the peninsula from the rest of Croatia; for trains you go via Slovenia.
The catamaran from Venice to Poreč, Rovinj, and Pula runs in summer and takes between three and four hours — slow but a fine way to arrive. Local buses connect all the main towns frequently; for the hill villages and wine country a car is genuinely useful.