The present contribution is the first in a series which will deal with forest health. It is designed for foresters in practice and aimed at a more efficientperformance of the Reporting, diagnostic and prognostic service in Slovenia. The most important harmful biotic factors (insects, fungi, bacteria,viruses, nematodes, small mammals) and abiotic factors (drought, fire, frost) will be presented. They will be arranged by their importance withregard to economic importance and by the host on which they most often appear. In the first contribution we present four species of bark beetles on Norway spruce (Picea abies): the eight-toothed spruce bark beetle (Ips typographus), six-toothed spruce bark beetle (Pityogenes chalcographus), smallspruce bark beetle (Polygraphus poligraphus) and small eight-toothed spruce bark beetle (Ips amitinus). A short description of their morphology, bionomy, a description of damages, possible misidentifications, hosts, causes of beginning or continuation of their outbreaks and their threat to forests isgiven. For the most important species control of population density and suppression are also described, and their most important natural enemies are listed.