The two-frequency problem of synchronization of the pulse train of a passively mode locked soliton laser to an externally injected pulse train is solved in the weak injection regime. The source and target frequency combs are distinguished by the spacing and offset frequency mismatches. Locking diagrams map the domain in the mismatch parameter space where stable locking of the combs is possible. We analyze the dependence of the locking behavior on the relative frequency and chirp of the source and target pulses, and the conditions where the relative offset frequency has to be actively stabilized. Locked steady states are characterized by a fixed source-target time and phase shifts that map the locking domain. to the external frequency for strong enough injection or small enough frequency mismatch, with a sharp threshold separating configurations with stable locked steady states from unlocked configuration. Injection locking in continuous wave (cw) lasers has been extensively studied experimentally and theoretically [2]. Injection locking is useful to stabilize the target laser versus phase diffusion and frequency drift, and can be achieved for very weak seeding provided the frequency mismatch is small enough. Strong injection leads to rich behavior, including bifurcations, multistability, excitability and chaos [2].If the laser is mode locked, and the injected signal is pulsed, the target laser pulse train can injection-lock to the source pulse train. Unlike cw synchronization that implies entrainment of a single optical frequency, pulse train synchronization entails entrainment of two frequencies, namely the pulse repetition rate and the pulse phase shift per round trip. Since these two frequencies determine the spacing and offset of the pulse train frequency comb, pulse train synchronization is equivalent to injection locking of the target laser frequency comb to the comb of the source. In this way a very high-quality standard frequency comb source, that can be quite weak, could impart its quality to the target laser, locking it to the standard and reducing imperfections, such as pulse noise and frequency drift. A natural application is when the source is carrier-envelope phase locked, where injection locking locks the carrier-envelope phase of the target. When the repetition rates of the source and target are not matched but rationally related, synchronization results with a target repetition rate that is the least common multiple of the source and free running target, facilitating generation of high-repetition rate frequency comb [3].Compared to cw injection locking, there are few studies of injection locking of mode locked lasers. The basic theory of the system was developed in [4] using soliton perturbation theory in the framework of the Haus master equation model. Margalit et al [4] derived estimates of the locking regime parameters, and studied numerically the pulse dynamics for some typical locked and non-locked parameters. The phenomenon was demonstrated experimentally in actively mode locked lasers in ...