Spain is a country in which a large part of the economy relies on tourism. For this reason it is hard that you will ever be too far away from some accommodation or that transportation will be impossible. Excluding the natural parks, you have a comprehensive road and railway network. All Regional capitals have an airport and though not all of them qualify as international airports, they do have regular domestic routes. Flying inside Spain can be sometimes cheaper than other kinds of transport, but to figure this out you should keep an eye on local and foreign airlines that operate domestic flights in the country and watch for offers.
From all the visitors that arrive in Spain, on third do it by air travel and arrive at the international airports available in the country: 'Barajas' in Madrid, 'El Prat' en Barcelona, Bilbao, Santiago de Compostela, Sevilla, Granada, Málaga, Almería, Alicante, Valencia, Palma de Mallorca, Girona and Ibiza.
The most busy domestic route is the one between Madrid and Barcelona called the 'Puente Aéreo' (the aerial bridge) transporting close to 2, 2 million passengers a year. Other very important routes are those that take visitors to the Balearic Islands and to the Canary Islands.
You can reach Spain by land through the borders of Portugal and France. There are 17 road access points from France, being the most busy that of 'Irún', on the western side of the border. Other relevant ones are in Cataluña like Puigcerdà, La Jonquera and Portbou, on the east side of the border.
There are internacional buses and trains that arrive to Spain directly from cities like Lisboa, London or Paris. Also by sea you can arrive in one of the many passenger ports along the Spanish coast. Once in you get there you might want to travel inside the country by car. All the major car rental companies (Avis, Hertz, Europcar) are available in Spain. Some cities have cheaper options like Easycar in Barcelona (in Barcelona, Madrid, Mallorca and Málaga) or some local car rentals like Pepecar.
Please remember to inform yourself about visas and documentation. In our guide you can obtain information concerning embassies, visas and customs.
Concerning local or city transport, it changes from one city to another. You have bus/subway options to reach ariports to center of towns if you want to skip the sometimes expensive taxis. If you take a taxi make sure to ask for a receipt of your payment ('un recibo' in Spanish) in case you have any claims to make later. Taxis have to display their rates, or usually have them written on a sticker on the window.
In big cities you can buy a 10 one-way ticket for a significant reduced price that should allow you to use bus or subway indistinctively. There are also monthly tickets for those staying for longer periods.