Important changes have occurred over the last decades in the clinical application of the strategies for posterior restorations - from amalgam to composites in direct restorations and from traditional resistance form crowns to adhesive partial restorations such as onlays. Despite much evidence available for these advances, there are still very few established guidelines for common clinical questions: When does an indirect restoration present a clinical advantage over a direct one? When should one perform adhesive cusp coverage such as an onlay? When to implement resistance form designs in adhesive restorations? Which conditions create limitations for adhesion so that a resistance form preparation with a stiffer material such as a traditional crown might be more appropriate? In order to provide clinical guidelines, the present authors consider five parameters to support and clarify decisions - Coverage of cusps, Adhesion advantages and limitations, Resistance forms to be implemented, Esthetic concerns, and Subgingival management - the CARES concept. In Part I of this three-part review article, the focus is on clinical decisions for partial adhesive restorations regarding indications for direct versus indirect materials as well as the need for cusp coverage and/or resistance form preparations based on remaining tooth structure and esthetics.