Can a List<t> be accessed by multiple threads?

If you can (if you can use .NET 4 that is), use BlockingCollection<T>:

Provides blocking and bounding capabilities for thread-safe collections that implement IProducerConsumerCollection<T>.

If not then encapsulate the list completely and add thread-safe methods that access the List<T>'s state. Don't make the reference to the list public or return it from any methods - always encapsulate the reference so you can guarantee that you are locking around all access to it.

A List<T> is not a thread-safe class but if you lock everytime you read/write to it there won't be any issues. According to the documentation:

Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

A List<T> can support multiple readers concurrently, as long as the collection is not modified. Enumerating through a collection is intrinsically not a thread-safe procedure. In the rare case where an enumeration contends with one or more write accesses, the only way to ensure thread safety is to lock the collection during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.

List<T> is not thread-safe generally. Having multiple readers will not cause any issues, however, you cannot write to the list while it is being read. So you would need to lock on both read and write or use something like a System.Threading.ReaderWriterLock (which allows multiple readers but only one writer).