The architect was a Spanish man called Antoni Gaudí. He designed it to look like it had grown out of the ground, like a forest of stone trees. Inside, the columns branch out at the top like real trees, and light pours through coloured windows so it feels like sunlight through leaves. Gaudí said he was 'copying nature, because nature is the best builder'.
Gaudí worked on the Sagrada Família for 43 years, until 1926. He didn't finish it, of course - it was always going to take much longer than one lifetime. He left detailed models so the builders who came after him could keep going. Some of his original plaster models were broken, so today's architects also work like detectives, figuring out what Gaudí would have wanted.
Building has been slow for lots of reasons. Spain went through difficult times in the 20th century when work stopped. The whole thing is paid for only by the tickets of people visiting - so it can only be built as fast as visitors come. Modern computers and 3D printers now help a lot, and builders hope to finish the very top tower by 2026.
When it is finished, the central tower will be 172.5 metres tall - just one metre shorter than the highest hill in Barcelona, because Gaudí said no human-made thing should be taller than what God made.