How can I debug a 'ReferenceError is not defined' when clicking an HTML button?

First, carefully examine the JavaScript code within your `onclick` event handler. Use your browser's developer tools (usually accessed by pressing F12) to set breakpoints and step through the code. Check if the variable/function is declared before it's used. Also, verify the scope – is it accessible from where you're calling it? Console.log() statements can help trace variable values and execution flow.