The IFAE is located in the Campus of the Autonomous University of Barcelona (Universitat Autònoma de Barcelona, UAB) in Bellaterra. Bellaterra is a town outside the Barcelona Metropolitan Area, but can be reached very easily from downtown Barcelona or the Airport by public transportation. See also maps from the University web.
The recommended method is to get to Plaça de Catalunya from the airport by train, by the "Blue Bus" or by taxi. The train (renfe) leaves the railway station close to the airport every 30 minutes and it costs about 2€; it gets to the downtown in less than 25 minutes. The Blue Bus (labeled "Aeroport") leaves the airport every 15 minutes and takes 20-25 minutes to arrive to Plaça de Catalunya (its end destination). The price is 3€. By taxi the drive takes about 15 to 20 minutes and the price should be of the order of 20€.
Once in Plaça de Catalunya the easiest way to get to the UAB is to take the train called Ferrocarrils de la Generalitat de Catalunya (FGC). Enter the Ferrocarrils underground station at the beginning of the "Ramblas", a street which everyone knows in Barcelona. The entry it is at the Eastern edge of Plaça de Catalunya, the "sea side", and it is common for the Ferrocarrils and the Metro.
Once there follow the sign of the Ferrocarrils. At the Ferrocarrils station there are trains departing to several end destinations. To get to the UAB take the train in the direction of Sabadell (line S2). Trains leave towards Sabadell every 12 minutes on working days, and at somewhat longer intervals on week-ends and holidays. The trip takes about 35 minutes and costs less than 2.5 € The name of the stations and the end destination (Sabadell) are displayed in luminous panels at the appropriate isle, and also on the side of the trains themselves. On working days there are a few trains which come to the UAB only, and they do not continue to Sabadell; they are labeled line S55.
Path from the FGC railway station to the institute
If you come to the Hotel Campus, it is easier to get off the train at the Bellaterra stop, just one station before UAB, and from there walk to the hotel. The station of Bellaterra is at the edge of the campus, about 300m away from the hotel. The walk is through a paved road going through the campus woods. There is always someone at the train station, and it is advisable to ask there for directions. From the UAB station one can also walk to the hotel, but it is further away, about 600m going up.
An alternative way of coming from the airport to the UAB is to take a taxi to the Ferrocarrils station of Sarrià, located at the end of "Via Augusta", and enter the train there. The taxi to the Sarrià Station takes the same time than to Plaça de Catalunya, but one is already almost halfway to Bellaterra by train.
Yet another way, is to take a taxi directly from the airport to the UAB Campus. The price should be of the order of 35 €.