Charles helps to debug a wide range of web problems. Below is a list of Charles’ features:

• Cookies are shown in the HTTP headers, so you can see exactly what cookies you are sending and receiving.
• Every request and response is recorded in Charles. Redirects that are often too quick to see when testing with a web browser can be seen in Charles. Requests from applications other than your web browser (such as Flash movies) can also be seen.
• Request and response sizes are shown in Charles, so you can see how big each request was.
• Assets loaded from an HTML page are recorded so that you can see how many images etc are loaded by a page, and where from.
• All files can be viewed, including JavaScript files, CSS files, HTML files etc.
• Mirror all responses to disk, recording your session.
• Blacklist sites so that requests are blocked.
• See the results of caching by seeing cached responses (304 Not Modified), and requests containing last modified dates (IfModifiedSince).
• Disable caching by removing cache related headers from requests and responses as they pass through Charles, ensuring that you are always requesting the latest file.
• See whether a cache has served your request by looking for cache-hit HTTP headers.
• View encrypted HTTPS traffic in plain text. Enables you to view requests and responses in plain text even when communicating with an SSL secured web server.
• Reveal unexpected requests, such as typos and 404s.
• View requested images.
• Bandwidth Throttle your web connection to a specified bytes/second speed, and millisecond latency. This enables you to simulate modem conditions on a high speed internet connection (bandwidth simulator)
• Spoof DNS name to ip mappings so that you can test a domain name before it has gone live. Very useful for testing your virtual hosting.
• Export to CSV all of the summary data captured by Charles for analysis and reporting in Excel
• Reverse proxy creates ports on the localhost that act as regular HTTP servers, but forward all requests to a specified web server.
• HTTP 1.1 support - including keep-alive, chunking and content-encodings including gzip, compress and deflate.
• External proxy support - configure an proxy for Charles to use to access the Internet
• NTLM authentication support (Windows NT Challenge Response Authentication / Integrated Windows Authentication)
• Automatic configuration of Windows Internet Proxy settings.
• Search all headers and bodies for keywords.
• Port forwarding support, allows any protocol to be forwarded by Charles and debugged.
• Unicode and other charset encoding/decoding support.
• AJAX debugging (Asynchronous Javascript and XML) and XMLHTTP debugging
• AMF parsing to help debug Flash Remoting
• Firefox proxy autoconfiguration
• Automatic Update Checking

Charles reveals everything that takes place on the web. See the screenshots page for examples of the above.