The overall aim of this thesis was to study the thinking strategies and clinical reasoning processes of registered nurses (RNs) and to implement and test a computerized decision support system (CDSS) integrated into the electronic health care record (EHR) to improve patient outcomes, i.e. to prevent pressure ulcers (PUs) and malnutrition among residents in nursing homes.  A think-aloud (TA) study with a purposeful sample of RNs (n=30) was conducted to explore their thinking strategies and clinical reasoning (Paper I). A quasi-experimental study with a convenience sample of residents (at baseline, n=491 and at follow-up, n=480) from nursing homes (n=15) allocated into two intervention groups and one control group was carried out in 2007 and 2009 (Paper II). In Paper III residents’ records were reviewed with three instruments. Nursing personnel (n=25) from four nursing homes that had used the CDSS for eight months were interviewed and the CDSS was tested by nursing personnel (n=5) in two usability evaluations (Paper IV). The results showed that the RNs used a variety of thinking strategies and a lack of systematic risk assessment was identified (Paper I). The proportion of malnourished residents decreased significantly in one of the intervention groups after implementing the CDSS, however there were no differences between the groups (Paper II). The CDSS resulted in more complete and comprehensive documentation of PUs and malnutrition (Paper III). The nursing personnel considered ease of use, usefulness and a supportive work environment as the main facilitators of CDSS use in nursing homes. Barriers were lack of training, resistance to using computers and limited integration of the CDSS within the EHR system (Paper IV). In conclusion, the findings support integrating CDSSs into the EHR in nursing homes to support the nursing personnel.