I found the older generation (retirement age) to be remarkably well-dressed in Spain any day of the week in the evening…nice slacks, dress shirts, jackets, and even ties for men. Women wear nice dresses. Younger men and women dress fashionably…very similar to what you’ll see in Paris or any other large metropolitan area. College-age kids wear jeans and tee shirts…whatever’s in…skinny jeans, athletic-cut shirts, etc. Sunglasses during the day are huge with younger people…seems like the bigger the better. I agree with everyone else…a tie isn’t really necessary, but you should dress up a little (better casual) if your desire is to fit in a little better for dinner in a restaurant and a stroll afterward when you're a tourist. If you’re just hitting tapas bars, I wouldn’t think it necessary to dress up at all, but you certainly could if you wanted to with no downside. I was just really struck by how well dressed the older folks are…more so than any other place I’ve visited.