If you're going to a festival like Primavera Sound, and especially if you're in a foreign country, it's really a good idea to know how to get around on public transport, particularly at the dreaded moment when you make the decision to go home because you've just noticed that the morning sun is starting to light up the sky. It's dawn, you're wrecked from dancing your feet off and you can't bear waiting in the taxi rank with not a single taxi in sight but a queue that stretches nearly back to where your bed is. Happily, there is an alternative in the free shuttle that Primavera Sound offers between the Parc del Fòrum and Plaça Catalunya.
The shuttle service will really come in handy in the hours the metro is closed, that is, Thursday 28 between midnight and 5am, and Friday 29 between 2am and 5am (note that the metro runs all night on Saturdays, including Saturday 30 during the festival). The shuttle comes every 10 minutes and you pay €2 per ticket. To avoid the horror of waiting in line only to be turned away at the door of the shuttle, know that you must buy a ticket before boarding the bus. Get your tickets at the booth set up next to the shuttle stop.
When the metro is running (Thursday 28, 5am to midnight; Friday 29, 5am to 2am; Saturday 30, non-stop all night), you'll likely find that the best way to get to and from the Fòrum. The closest metro stop is El Maresme/Fòrum on Line 4 (the yellow line). The Trambesòs (tram lines T4, T5 and T6) also drops you or picks you up at the Fòrum from early in the morning (around 5am) and stops at spots like Glòries or the commuter stop Sant Adrià del Besòs.
As for while you're inside the festival venue, if walking the considerable distances between some of the stages makes you want to just lie down and listen to anything that plays near your patch of green, you can keep going with the Transfer service linking the esplanade area (Ray-Ban and ATP) with the stages on the marine platform (Heineken and Spring).
If you're a festival-goer with reduced mobility, you'll can access the venue via the guest tent, and you're invited to use the reserved areas at each stage. When you enter the Fòrum, just be sure you request the bracelet that gives you access to the reserved areas. (Note that one person is permitted to accompany anyone who requires access to these areas.)
Oh, one more thing: If you're a football fan and can't bear the thought of missing the final of the Copa del Rey between Barça and Athletic Bilbao, don't worry about how you'll get out of the Fòrum, where you'll go and how you'll get back in – on Saturday 30 there will be a big screen set up in the food area where you can watch the match. Now that's a festival with everything in mind.