Hey, I'm not sure if this is a logback or slf4j issue, so sorry if it's a slf4j issue. My question is posted on stackoverflow ( http://stackoverflow.com/questions/31772577/org-slf4j-helpers-substitutelogg... ) but I will paste it here: I've seen some questions very similar to this one ( http://stackoverflow.com/questions/15483309/org-slf4j-helpers-noploggerfacto...)), but none of them received a good answer, or at least one that explained or solved this problem I was able to create a very small project (basically 2 Scala classes - each with a logger - and 2 test classes) with a similar structure to my real project where I found the problem. The example is available here: https://bitbucket.org/pedrorijo91/logger-exp The problem happens when I run `sbt test`, resulting in the following error: {14:43:41} (#47) ~/Desktop/logger-exp/log-exp@pedrorijo(master) $ sbt test Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=384m; support was removed in 8.0 [info] Loading global plugins from /Users/pedrorijo/.sbt/0.13/plugins [info] Loading project definition from /Users/pedrorijo/Desktop/git/scala/logger-exp/log-exp/project [info] Set current project to log-exp (in build file:/Users/pedrorijo/Desktop/git/scala/logger-exp/log-exp/) [info] HelloTest: [info] Exception encountered when attempting to run a suite with class name: HelloTest *** ABORTED *** [info] java.lang.ExceptionInInitializerError: [info] at HelloTest$$anonfun$1.apply$mcV$sp(HelloTest.scala:8) [info] at HelloTest$$anonfun$1.apply(HelloTest.scala:8) [info] at HelloTest$$anonfun$1.apply(HelloTest.scala:8) [info] at org.scalatest.Transformer$$anonfun$apply$1.apply$mcV$sp(Transformer.scala:22) [info] at org.scalatest.OutcomeOf$class.outcomeOf(OutcomeOf.scala:85) [info] at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104) [info] at org.scalatest.Transformer.apply(Transformer.scala:22) [info] at org.scalatest.Transformer.apply(Transformer.scala:20) [info] at org.scalatest.FunSuiteLike$$anon$1.apply(FunSuiteLike.scala:166) [info] at org.scalatest.Suite$class.withFixture(Suite.scala:1122) [info] ... [info] Cause: java.lang.ClassCastException: org.slf4j.helpers.SubstituteLogger cannot be cast to ch.qos.logback.classic.Logger [info] at com.example.Hello$.<init>(Hello.scala:8) [info] at com.example.Hello$.<clinit>(Hello.scala) [info] at HelloTest$$anonfun$1.apply$mcV$sp(HelloTest.scala:8) [info] at HelloTest$$anonfun$1.apply(HelloTest.scala:8) [info] at HelloTest$$anonfun$1.apply(HelloTest.scala:8) [info] at org.scalatest.Transformer$$anonfun$apply$1.apply$mcV$sp(Transformer.scala:22) [info] at org.scalatest.OutcomeOf$class.outcomeOf(OutcomeOf.scala:85) [info] at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104) [info] at org.scalatest.Transformer.apply(Transformer.scala:22) [info] at org.scalatest.Transformer.apply(Transformer.scala:20) [info] ... SLF4J: The following set of substitute loggers may have been accessed SLF4J: during the initialization phase. Logging calls during this SLF4J: phase were not honored. However, subsequent logging calls to these SLF4J: loggers will work as normally expected. SLF4J: See also http://www.slf4j.org/codes.html#substituteLogger SLF4J: com.example.Hello$ 14:43:52.846 [pool-6-thread-4-ScalaTest-running-WorldTest] INFO com.example.World$ - LOGGING [info] WorldTest: [info] - test [info] Run completed in 456 milliseconds. [info] Total number of tests run: 1 [info] Suites: completed 1, aborted 1 [info] Tests: succeeded 1, failed 0, canceled 0, ignored 0, pending 0 [info] *** 1 SUITE ABORTED *** [error] Error during tests: [error] HelloTest [error] (test:test) sbt.TestsFailedException: Tests unsuccessful [error] Total time: 1 s, completed Aug 2, 2015 2:43:52 PM but if I run each of the tests individually through: `sbt "testOnly HelloTest"` and `sbt "testOnly WorlTest"` I get no error. As far as I've seen this is not just a Scala issue, it seems that in Java is also possible to happen. Furthermore, I've read that during initialization the loggers are substituted by a Substitute Logger ( http://www.slf4j.org/codes.html#substituteLogger). But I can't understand: 1. why it only happens in tests, and when running both tests? 2. why it happens? 3. how to debug/solve this problem? As I said before, I've created a repository with a working example (working, meaning with the error happening) here: https://bitbucket.org/pedrorijo91/logger-exp -- Kind Regards, Pedro Rijo