Techniques and tools are described for flattening nested streams. For example, a stream flattening tool processes nested streams on a stack to traverse the nested streams. The tool produces a flattened stream that enumerates the elements of the nested streams. In one embodiment, the tool initially pushes a stack frame onto the stack for a stream representing all of the nested streams. As the tool iterates across the “active” stream having the frame on the top of the stack, the tool may encounter and produce elements in the “active” stream. Or, the tool may encounter a nested stream and push a stack frame for the nested stream onto the stack, then continuing by iterating across the new active stream. The tool pops the stack frame for the active stream from the stack when the end of the stream is reached, or potentially earlier if handling a nested tail stream.