The PATH and CLASSPATH are two most important environment variables of Java environment which is used to find the JDK binaries used to compile and run Java in windows and Linux and class files which are compiled Java bytecodes. From my personal experience I can say that PATH and CLASSPATH are two most problematic thing for beginners in Java programming language due to two reasons; first because in most of Java courses nobody tell details of what is a PATH and CLASSPATH, What do PATH and CLASSPATH do, What is meaning of setting PATH and CLASSPATH, What happens if we do not set them, Difference between PATH vs CLASSPATH in Java or simply How Classpath works in Java etc. These basic question which answers most of the details about PATH and CLASSPATH in Java are mostly not answered until Java programmer itself acquire these knowledge, Things may be changed nowadays but important of PATH and CLASSPATH is still high. Most common cause of dreaded error like java.lang.NoClassDefFoundError and java.lang.ClassNotFoundException is either incorrect or misconfigured CLASSPATH in Java.