The US Supreme Court reversed itself on whether a jury must decide facts that decide a mandatory minumum sentence or if a judge can find those facts as "sentencing factors."  Previously, the court decided Harris v. United States, 536 U. S. 545 (2002) and held the judge could constitutionally decide facts in support of a higher mandatory minimum sentence.   

In Alleyne v. United States, No. 11-9335 (Jun. 17, 2013) the court expressly overruled Harris and held that the legislature cannot force a judge to impose a higher mandatory minimum sentence unless a jury finds the requisite statutory factual predicates. 

The case means that in any crime involving a mandatory minimum must now allege those facts in the criminal complaint and prove those facts to the jury before the judge is compelled to sentence the defendant to the greater mandatory minimum.  If the prosecution fails to properly plead or prove the elements for the enhanced penalty, the court may still exercise its discretionary authority to impose the higher sentence, but is not compelled to do so.

Note: In a footnote, the court affirms its prior decision allows judges to determine prior offense enhancements, as stated in Almendarez-Torres v. United States, 523 U. S. 224 (1998). 

The full case is available here.