Initial SessionFactory creation failed.java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory

PROBLEM:
If working with Hibernte, you got this message:
Initial SessionFactory creation failed.java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory 

SOLUTION: 
solve the problem adding this two jar files to the project's library:
   slf4j-api-1.5.10.jar
   slf4j-simple-1.5.0.jar

You find those jars on  http://www.slf4j.org/manual.html 

Go to the download page and download the file "slf4j-X.Y.Z.zip" (or the tar file).
Be sure that the version matches you project's environment.
How?
Simple: check the project jars' versions, compare and try! : )