This report provides guidance on how to move from current practice to recommended practice in Life Cycle Impact Assessment. It is composed of three complementary parts elaborated in the first task force (TF1) of the LCIA programme, with contribution of the other three task forces: 1. The first part concentrates on midpoint categories. It defines a consistent and systematic process to evaluate different category indicators within an LCIA midpoint category, and to recommend an adequate indicator or set of indicators. It then applies this process to provide recommendations for acidification modeling: Six currently used methods are compared to provide a practical illustration and to demonstrate its applicability. This application on acidification will serve as a template to provide guidance for the selection of indicators for other midpoint categories. 2. Based on the analysis of impact pathways in individual midpoint impact categories, the second part describes some possible damage categories and indicators. It constitutes a starting point to assess environmental relevance across the different midpoint impact indicators and a preliminary basis to ensure a consistent and common approach to damage modelling. It aims covering all emission related damages categories, where there is sufficient knowledge available. 3. The third part focuses on aspects of location and time as they affect impact modeling, i.e. spatial and temporal differentiation. This section first establishes basic principles determining a) the need for spatial or archetype differentiation in the different impact categories, b) possible ways to address this, and c) the consequences for LCI and LCIA modelling.