[en] The accreditation of an Internal Dosimetry Service (IDS) according to ISO/IEC 17025 Standard is a challenge. The aim of this process is to guarantee the technical competence for the monitoring of radionuclides incorporated in the body and for the evaluation of the associated committed effective dose E(50). This publication describes the main accreditation issues addressed by CIEMAT IDS regarding all the procedures involving good practice in internal dosimetry, focussing in the difficulties to ensure the traceability in the whole process, the appropriate calculation of detection limit of measurement techniques, the validation of methods (monitoring and dose assessments), the description of all the uncertainty sources and the interpretation of monitoring data to evaluate the intake and the committed effective dose. CIEMAT Internal Dosimetry Service (IDS) has developed and implemented a quality system based on ISO/IEC 17025 to ensure compliance with the general requirements of this reference standard. The development of documentary support according to this quality system permitted to standardise the systematic activities performed within the whole body counter and in vitro bioassay laboratories as well as the procedures carried out by qualified staff in charge of internal dose assessment. There was no previous experience in the accreditation of other internal dosimetry services in Spain. Then, requirements from the national regulatory body (Nuclear Safety Council, CSN) and national accreditation entity (ENAC) have been considered. The main concerns were to guarantee the traceability in the whole process and to avoid possible charge of interpretation or subjectivity in the methodology of dose assessment due to intakes of radionuclides when calculating from monitoring data. All the related international standards dealing with internal dosimetry were taken into account: ISO 28218 'Performance criteria for radiobioassay', ISO 27048 'Dose Assessment for the monitoring of workers for internal radiation exposure' and ISO 20553(4) 'Monitoring of workers occupationally exposed to a risk of internal contamination with radioactive material'. Appropriate and validated commercial software implementing current ICRP biokinetic and dosimetric models was approved as tool for dose assessment. CIEMAT IDS was accredited according to ISO/IEC 17025 in 2012 as demonstration of technical competence for monitoring and dose assessment due to internal exposures, considering the methods described as follows: In vivo monitoring of (a) gamma emitters in the total body using a FASTSCAN counter or one NaI(Tl) detector inside a shielded room, (b) actinides in the lungs with four LE Ge detectors and (c) radioiodine ("1"2"5I, "1"3"1I) in the thyroid with detection systems mentioned before. In vitro monitoring of (a) alpha emitters in urine and faeces by alpha spectrometry, (b) beta emitters ("9"0Sr, "3H, "3"2P, "1"4C, "3"5S) in urine by liquid scintillation counting (LSC) and (c) uranium in urine using kinetic phosphorescence analyser (KPA). ICP-SFMS technique is in process of approval and accreditation for the measurement of uranium, thorium and plutonium in urine. Evaluation of committed effective dose E(50) from monitoring data, applying ISO standards, ICRP publications and IDEAS guidelines (GL) (EURADOS Report 2013-01)(5). (authors)