Sufficient Conditions for Quadrilaterals to Be Parallelograms

What are the sufficient conditions so you can conclude that a quadrilateral is a parallelogram? You may consider the converses of the theorems on parallelogram. Are these converses of theorems always true?

Theorem. Converse of the following: Opposite sides of a parallelogram are congruent.
If two pairs of opposite sides of a quadrilateral are congruent, then the quadrilateral is a parallelogram.