Designing and evaluating a web-based behavioral intervention system is not a trivial task. Such a system has become more and more sophisticated, and understanding the impacts of a web-based intervention often requires multi-phased, longitudinal evaluation studies. In this paper, we would like to share lessons that we have learned while designing and evaluating a web-based dietary intervention system for patients with coronary heart disease, called “Food for the Heart.” The three main lessons are as follows: 1) Health intervention requires multi-faceted support; 2) Design should mimic the existing mental model; and 3) Different evaluation methods serve different purposes. In addition, we share some minor technical lessons learned while implementing the website. These lessons may be helpful when designing and evaluating a web-based healthcare intervention system.