Do ASP.NET developers really need to be concerned with thread safety?

There are certain objects in addition to static items that are shared across all requests to an application. Be careful about putting items in the application cache that are not thread-safe, for example. Also, nothing prevents you from spawning your own threads for background processing while handling a request.

There are different levels of ASP.NET Developers. You could make a perfectly fine career as an ASP.NET Developer without knowing anything threads, mutexes, locks, semaphores and even design patterns because a high percentage of ASP.NET applications are basically CRUD applications with little to no additional business logic.

However, most great ASP.NET Developers which I have come across aren't just ASP.NET Developers, their skills run the gamut so they know all about threading and other good stuff because they don't limit themselves to ASP.NET.

So no, for the most part ASP.NET Developers do not need to know about thread safety. But what fun is there in only knowing the bare minimum?

Only if you create, within the processing stream for a single HTTPRequest, multiple threads of your own... For e.g., if the web page will display stock quotes for a set of stocks, and you make separate calls to a stock quote service, on independant threads, to retrive the quotes, before generating the page to send back to the client... Then you would have to make sure that the code you are running in your threads is thread-safe.