Loading page in WPF

I came here to add that there are many ways to load the pages into the frame:

By setting the source (as @Shift mentioned)

frame1.Source = new Uri("Page1.xaml", UriKind.RelativeOrAbsolute);

By setting the Content:

frame1.Content= new Page1();

By using the NavigationService:

frame1.NavigationService.Navigate(new Page1());

Adding a frame and setting the source for the frame like makes my day :)

frame1.Source = new Uri("Page1.xaml", UriKind.RelativeOrAbsolute);