Objective: This paper reviewed the current understanding of the psychopathology of autism in very young children. Tools that were developed based on this psychopathology profile, and used for screening and disgnostic assessment in this age group, were also introduced. Method: Literatrue reviews and research comparions were conducted. Four topics were reviewed in this paper: (1) the psychopathology in young children with autism below age of three; (2) representative screening and diagnostic assessment tools for young children with autism; (3) the strength and weakness of these tools; and (4) suggestions for future development of these tools in Taiwan. Conclusion: Literature on psychopathology in 0 to 3 year-old children with autism repetitively demonstrated the impairment of several socio-communicative abilities. Among them, joint attention, imiation, pretend play and emotional sharing were consistently found to be impaired in young children with autism. While the abnormality of the sensory reaction, and the restricted repetitive and stereotyped behaviors in the first three years of age were not consistently reported. Based on research findings of socio-communicative deficits in very young children with autism, some screening and diagnostic assessment tools were developed for research and clinicla use. CHAT (Checklist for Autism in Toddlers), M-CHAT (Modified-CHAT), STAT (Screening Tools for Autism in Toddlers), and CSBS DP (Communication and Symbolic Behavior Scales: Developmental Profile) are for screening, and CARS (Childhood Autism Rating Scale), ADI-R (Autism Diagnostic Interview-Revised), and ADOS (Autism Diagnostic Observation Schedule) are for diagnostic assessment [AC1]. Most of these instruments have adequate reliability and validity. Clinicians can selectively use them th meet their clinical needs. The development of these screening and diagnostic assessment instruments is a good example connecting theoretic research and clinical application. However, some shortcomings of these instruments are noteworthy. First, all of the screening tools screen for the impairment in social reciprocity and communication but not the abnormality in sensory response and the restricted repetitive stereotyped behaviors which are one of the three main features of autism. Second, all of the diagnostic instruments, i.e., ADOS and ADI-R, do not provide a clear subclassification of the autism spectrum disorders, i.e., Asperger's disorder or the PDDNOST [AC2] (Pervasive Developmental Disorders Not Otherwise Specified). Third, all of the previous instruments were developed by the classical test theory;; new methodology from modern test theory needs to be used for solving the scaling issues. Forth, certification training workshops for the diagnostic tools, i.e., ADI-R and ADOS, are being provided to clinicians and researchers with high fees. The promotion for the use of ADI-R and ADOS is very much commercialized and monopolized, which is not consistent with the academic tradition. Modifications of the available tools or the development of new tools are necessary to overcome the first two weakness. While the nonprofit use of the tools shall be publicized to overcome the third weakness. Taiwan is in the beginning phase of developing early screening and identification tools for very young children with autism. Suggestions for the development of such tools in the future are provided.