Andrea Posilicano Poincar\'e-invariant Markov Processes and Gaussian Random Fields on Relativistic Phase Space (163K, postscript) ABSTRACT. We give a complete characterization, including a L\'evy--It\^o decomposition, of Poincar\'e--invariant Markov processes on $ H^1_+\times M^2$, the relativistic phase space in 1+1 space--time dimensions. Then, by means of such processes, we construct Poincar\'e-- invariant Gaussian random fields, and we prove a ``no--go'' theorem for the random fields corresponding to Brownian motions on $H^1_+\times M^2$.