One of the main advantages that on-line working provides is the ability to interact with a running program. Interaction at run-time is important from two standpoints. Firstly, it enables the programmer to control the action of the program by providing suitable data as the execution progresses. Secondly, and most importantly, given the right facilities, it enables a programmer dynamically to debug the program. One of the most popular languages designed explicitly for on-line working is BASIC. Although one is able to interact in the first sense, most implementations fail to give any run-time debugging facilities to the BASIC user. This paper describes the range of run-time debugging facilities provided on an implementation of BASIC at Hatfield Polytechnic on an ICL 803.