Serial.print from multiple files of one sketch

I have created a project (sketch) which contains multiple .c and .h files - code is compiling properly - but I want to chk in values generated at different steps in my code. I am using Serial.print for the same -

I am getting error saying Serial is not declared -

I have noticed that from main file serial works - but from any other c file of the sketch gives error - how to solve this?

Some #includes are apparently missing from secondary .c files.