How do I open a second window from the first window in WPF?

Write your code in window1.

private void Button_Click(object sender, RoutedEventArgs e){    window2 win2 = new window2();    win2.Show();}

When you have created a new WPF application you should have a .xaml file and a .cs file. These represent your main window. Create an additional .xaml file and .cs file to represent your sub window.

MainWindow.xaml

<Window x:Class="WpfApplication2.MainWindow"    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"    Title="MainWindow" Height="350" Width="525">    <Grid>        <Button Content="Open Window" Click="ButtonClicked" Height="25" HorizontalAlignment="Left" Margin="379,264,0,0" Name="button1" VerticalAlignment="Top" Width="100" />    </Grid></Window>

MainWindow.xaml.cs

public partial class MainWindow : Window{    public MainWindow()    {        InitializeComponent();    }    private void ButtonClicked(object sender, RoutedEventArgs e)    {        SubWindow subWindow = new SubWindow();        subWindow.Show();    }}

Then add whatever additional code you need to these classes:

SubWindow.xamlSubWindow.xaml.cs

private void button1_Click(object sender, RoutedEventArgs e){    window2 win2 = new window2();    win2.Show();}