Axillary brachial plexus block is one of the most popular and widely used approaches for brachial plexus blocks. Its main advantages are its versatility and high safety. Brachial block facilitates analgesia for the distal arm, elbow, forearm and hand. Numerous upper limb procedures, particularly orthopedic ones, can be carried out under axillary block. Axillary block is well suited for the ultrasound-guided technique. Because the brachial plexus in the axillary region is located superficially, the nerves, block needle, and local anesthetic spread are all relatively easy to visualize. A high-frequency linear probe can be used during block procedure, so the quality and resolution of the ultrasound images are excellent. An important feature of the axillary approach is its high level of safety. In the axillary area, there are no anatomical structures other than vessels, to which damage during block placement could pose a risk for the patient. For this reason, axillary block is one of the techniques that are recommended for learning ultrasound-guided regional anesthesia. This paper summarizes anatomical fundamentals and provides basic sonoanatomic knowledge that is essential for successful ultrasound-guided axillary block.Key words: regional anesthesia, peripheral nerve block, brachial plexus; regional anesthesia, peripheral nerve block, axillary block; ultrasound, sonoanatomy Anaesthesiology Intensive Therapy 2015, vol. 47, no 4, 409-416 Axillary brachial plexus block is one of the most popular and widely used techniques for brachial plexus blocks [1]. This approach is very universal and safe, and it allows analgesia for the distal arm, elbow, forearm and hand. Numerous upper limb procedures, in particular orthopedic ones, could be carried out under axillary block. Axillary block is well suited for ultrasound-guided techniques. The brachial plexus in the axillary region is located superficially, so the nerves, block needle and local anesthetic spread are relatively easy to visualize. A high-frequency linear probe can be used during block procedure, so the quality and resolution of ultrasound image are excellent. This approach is also very safe because in the axillary area there are no anatomical structures other than vessels, which damage during block placement could put a patient at risk. For this reason, axillary block is one of the techniques that are recommended for physicians learning ultrasound-guided regional anesthesia [2]. This paper is the first part of a review of the axillary brachial plexus block that summarizes the anatomical fundamentals and basic sonoanatomic knowledge that are essential for successful ultrasound-guided axillary block. Part two presents the techniques and possibilities for performing the axillary brachial plexus block when using ultrasonography.