My experience regarding sizing is different from the two answers below. I ordered based on these answers and found the boots way too big. I'm a 13 regular shoe size, but wear a Mondo 29 in a Scarpa F3. Based on these comments I decided to order a 30 in Dynafit, and the thing is HUGE. In conclusion, for me the Scarpa and Dynafit sizing are very similiar in Mondo sizing. 29=29.I have found that the Dynafit sizing system fits more true than other manufacturers. Meaning, if you are a 10.5 shoe size, go with the 28.0 Mondo. I tried on the ZZero C4 and the ZZues by Dynafit and I fit much better into the 28.0 Mondo shell. My foot was cramped and it hurt after 5 minutes of putting the 27.0 on. In comparison, the 27.0 Shell in the Black Diamond factor was a much better fit while the 28.0 was too large. It depends on the manufacturer's shell sizing and the foot shape of the individual. But like I said, in my personal experience, the Dynafit boots fit more true to your actual foot size. So if you are a 10.5 and you are looking into these boots, then go with the 28.0. If anyone else has a different opinion then please chime in. Just wanted to add my comment, I also am 10.5 I just bought these in a 28. They fit perfect. In my true alpine boots I have always worn a 27.5 but the Dynafit 27.5 was just a bit to snug. The boots are great, plenty stiff. I had been skiing a old T2 and was concerned about enough stiffness in a AT boot, but these are more stiff than my alpine boots.