Preventive methods of plant protection used currently require a significant number of treatments throughout the season. Research results indicate a possibility of reduction of the number of treatments by halfwithout serious loss of effectiveness if the attack of pathogens has been recorded early enough. Limiting treatments to truly necessary ones means conserving chemicals, fuel and labour and consequently substantial financial savings for the farm. However, early warning requires full analysis of many factors that influence incidence, development and harmfulness of diseases, pests and weeds in relation with their impact on the yield. Such an analysis is beyond the power of the farmer alone. Appropriate models have to be used that utilize weather data and field observations in real time. At the present, due to frequent updating of the databases and sharing of the system by many users, in integrated plant protection the most efficient seem to be internet decision support systems. As of 2000 the Institute of Plant Protection in Poznań together with the Institute Soil Science and Plant Cultivation in Puławy in cooperation with the Danish Institute of Agricultural Sciences conduct a joint research project on development and implementation of an Internet Decision Support System for Integrated Plant Protection in Poland. Various modules of the system are already available on the Internet. Much like similar systems operating abroad, the Polish DSS also takes advantage of weather data utilized in disease models (the weather module). Nevertheless, an important addition to the system are strategically relevant data such as values and statistic distributions of elements of climate and potential yields (the agroclimate module), operation sheets, exploitation data and prices (the technology module) etc., which are important to the formulation and adoption of a particular line of action and risk evaluation in economic terms. When all modules are operational, the information relevant to decision making will be derived from on-line analyses based on cost calculation of different variants of plant protection applicable to the current situation.