Webdriver: java.lang.OutOfMemoryError: Java heap space

java.lang.OutOfMemoryError: Java heap space

1. My tests seem to be executing for a long while but then i get the listed error.
2. For example 50 out of the 150 tests will pass and then I get the following error and then the rest of the 100 tests will be skipped by TestNG when in turn they should also pass or fail etc.

thanks for your help