Update 2018
There is no need for extra CSS, and there are multiple centering methods in Bootstrap 4:
text-center for center display:inline elementsmx-auto for centering display:block elements inside display:flex (d-flex)offset-* or mx-auto can be used to center grid columnsjustify-content-center on row to center grid columnsmx-auto (auto x-axis margins) will center inside display:flex elements that have a defined width, (%, vw, px, etc..). Flexbox is used by default on grid columns, so there are also various centering methods.
In your case, you can simply mx-auto to the cards.