The discovery of mode-locking via saturable absorbers has led to optical femto-second pulses with applications ranging from eye-surgery to the analysis of chemical reactions on ultra-short timescales. In the frequency domain a train of such optical pulses corresponds to a frequency comb (equidistant optical laser lines spaced by the pulse repetition rate), which find use in precision spectroscopy and optical frequency metrology. Not relying on mode-locking, frequency combs can also be generated in continuously driven high-Q Kerr-nonlinear optical microresonators via cascaded four-wave mixing. Over the past years these Kerr-combs have been demonstrated in a variety of microresonator geometries. Applying a pulse-shaping mode locking mechanism, could enable compact femto-second pulse generators. However, conventional saturable absorbers are challenging to apply to microresonators, as they affect the high-quality-factor. Here, we report on passive mode-locking in microresonators without saturable absorber. This mode-locking is achieved via soliton formation supported by the balance between anomalous dispersion and Kerr-nonlinearity. The transition to and between different soliton states manifests itself as discrete steps in the resonator transmission. Numerical modeling of the nonlinear coupled mode equations in combination with an analytical description allows the identification of mode-locked regimes. Experimentally, we observe the generation of pulses with 200 fs duration. Equally important low noise and smooth comb spectra are achieved via single soliton states. In the time domain the presented results open the route towards compact femto-second sources, where the broadband parametric gain in principle allows for sub-cycle pulses. Moreover, femto-second pulses in conjunction with external broadening provide a viable route to microresonator RF-to-optical links.