Do they need to have the same frequency or amplitude? Should they pass through a given space at the same time? Should they have the same sources?
For two waves to interfere (by which I mean that they produce a stable interference pattern on a screen) they must be coherent i.e., they must maintain a constant phase difference and have same frequency.
However, the explanation above is naive because the real situation is a bit more complicated due to the fact that no source emits perfectly monochromatic waves. I'll try to give a more detailed answer.