Tropical
differential equations are introduced
and an algorithm is designed which tests solvability of a system of tropical linear
differential equations within the complexity polynomial in the size of the system
and in the absolute values of its coefficients. Moreover, we show that there exists a minimal solution,
and the algorithm constructs it (in case of solvability). This extends a similar complexity bound established for tropical linear systems. In case of tropical linear
differential systems in one variable a polynomial complexity algorithm for testing its solvability is designed.
We prove also that the problem of solvability of a system of tropical non-linear differential equations in one variable is NP-hard, and this problem for arbitrary number of variables belongs to NP. Similar to tropical algebraic equations, a tropical differential equation expresses the (necessary) condition on the dominant term in the issue of solvability of a differential equation in power series.