Hi I am not a newbie exactly but have not been using Arduino for very long. I really like it and the cost is hard to beat! One issue that came up lately is modifying library files. In this case it is an add-on library OneWireSlave. This library includes OneWireSlave.h and OneWireSlave.cpp .I came across some modified files at GitHub and needed to add them to the library. So I just copied them to the library and overwrote the existing files. For some reason I could not get the sketch of interest to compile successfully. I suspected some issue with the .h file from the compile errors but no amount of downloading and copying would change anything. I deleted the library but Arduino said the library still existed. The compiler still said it was getting the .cpp and .h from the library that I deleted How so??? I began searching for the two library files and found that they existed in the temp folder where arduino does the compile. Tell me if I am wrong but it seemed that these files were being used instead of the files from the non existent folder. Now I'm really scratching my head. I also read something about .o files that are produced and used each time a compile is done. So after deleting the .h, .cpp and .o files in the temp folder I did a verify and got the error "file does not exist". Good, just what I expected all along. I put the .h and .cpp files in my project folder and referenced them directly in the sketch and it compiled perfectly. What is going on with the library and how it is used and modified??? Please help me understand.