Certainly, the Houses of Parliament is a must see. All tourists take photos from the Westminster Bridge or the opposite bank of the Thames. My photo is from the inside of a pub across the street. If you are in London during the Summer Opening, you should take the opportunity to visit the interior of the Parliament building (recently, there are also tours on Saturdays throughout the year, I think). It is worthwhile despite the long queues at the ticket counter. But I would recommend to order a ticket in advance via this website. This was very convenient, we just had to be at the entrance about 15 minutes prior to the start of the guided tour. We took a tour in English, but there are also tours in other languages. The tour includes the most important areas of the Palace: Westminster Hall, the Robing Room, the Royal Gallery, Lords Chamber, Central Lobby, and the Commons Chamber. It was great to see all these famous rooms that are so full of history. The Houses of Parliament are very different from other parliament buildings in Europe, in particular the Common Chamber. It is a relatively small rectangular room and the two political parties sit opposite each other. The tour was one of the best guided tours I've ever attended. Our guide was excellent, he told many interesting facts about the building, the English history, and he described in detail the traditional State Opening of Parliament. This is a traditional ceremony, full of historical references and quirky rituals, as it is typical for England. It is the only occasion on which the monarch enters the Palace of Westminster. Before the ceremony, there is a ritual search in the cellars of the Palace. This is done in remembrance of the Gunpowder Plot in 1605. It is also a tradition, that a member of the Common Chamber stays as a hostage at Buckingham Palace during the monarch is in the Parliament, to ensure the safe return. I love such stories. Westminster was one of many highlights of our two weeks in London.