Spreadsheet programs, artifacts developed by non-programmers, are used for a variety of important tasks and decisions. Yet a significant proportion of them have severe quality problems. To address this issue, our previous work presented an interval-based testing methodology for spreadsheets. Interval-based testing rests on the observation that spreadsheets are mainly used for numerical computations. It also incorporates ideas from symbolic testing and interval analysis. This paper addresses the issue of efficiently debugging spreadsheets. Based on the interval-based testing methodology, this paper presents a technique for tracing faults in spreadsheet programs. The fault tracing technique proposed uses the dataflow information and cell marks to identify the most influential faulty cell(s) for a given formula cell containing a propagated fault.