In recent years, spins of merging black holes have been relatively accurately measured based on their gravitational-wave signals. Their spins are generally low, with the estimated average effective spin parameter as low as 0.06.

On the other hand, spins of many accreting black-hole binaries have been measured to be high, some close to the maximum spin parameter of 1, e.g. greater than 0.9985 at 3 sigma in Cyg X-1. I will present our recent results regarding this discrepancy.
In particular, I will discuss possible systematic effects affecting the spin measurements in accreting systems and whether they can be reconciled with those based on gravitational waves.