We investigate possibilities for detecting heavy Majorana neutrinos (N′s) in e+e- at LEP200 and future linear colliders. We concentrate on the processes where the pairs of intermediate heavy N′s produce a clear signal of total lepton number violation (e+e- → NN → W+l-W+l′-). Such a signal is not possible if the heavy neutrinos are of Dirac nature. Our approach is general in the sense that the intermediate N′s can be either on shell or off shell. Discussion of the relative numerical importance of the s and the t + u channels of the NN production is also included. © 1999 The American Physical Society.