Among the six-membered nitrogen-containing heterocycles the 1,2,4-triazines have attracted the attention of chemists for a long time. This is due to the fact that many 1,2,4-triazine derivatives are biologically active and are used in medicine and agriculture. Moreover, 1,2,4-triazines are of chemical interest. They proved to be very susceptible to attack by all kinds of nucleophiles, leading to the addition, and subsequently either substitution or cyclization, and ring transformation. This continuing and lively interest in this area of heterocyclic chemistry has produced a great number of publications and patent applications. This chapter concentrates on the behavior of monocyclic 1,2,4-triazines in reactions with C-, N-, 0-, and S-nucleophiles, because the important features of general character established for simple 1,2,4-triazines can evidently be applied to other derivatives including condensed 1,2,4-triazines.