Mainstream diagnostic classification systems such as Diagnostic and Statistical Manual of Mental Disorders (DSM) and The International Classification of Diseases (ICD) are facing a growing number of challenges. Alongside some improvements in reliability, categorical classification failed to explain high rates of comorbidity, heterogeneity of phenotypes within the same diagnostic category or difficulties that cause significant functioning impairments and suffering but do not meet diagnostic criteria for any defined disorder. At the same time, most practicing clinicians set aside diagnostic categories and use an eclectic approach when creating interventions for individual patients that include comorbidity and address subsyndromal or prodromal states. It was thus realized that diagnostic classification systems may not adequately reflect the real nature of mental health problems and that although they bear some usefulness it must not be forgotten that mental health problems are not the same as diagnoses. In recent years, a growing number of researchers and practitioners are putting aside traditional diagnostic approaches to mental health problems and are moving toward a transdiagnostic approach in understanding psychopathology and mental health in general. This approach cuts across diagnostic categories and focuses on identifying the core maladaptive processes that underpin a broad array of psychopathology and developing interventions that target these processes. Besides transdiagnostic factors and processes that are underlying vulnerability, in the context of resilience research, attention is recently given to identification of protective processes with transdiagnostic effects. Transdiagnostic field in its modern form began to emerge relatively recently, but since first studies at the beginning of the century, field rapidly evolved and from just a few scientific studies containing the term “transdiagnostic” is now counting almost 1500 scientific articles with more than a third of the number in the last three years. The aim of this paper is to synthesize existing literature in the field of transdiagnostic approaches to mental health to make it more accessible for interested professionals and practitioners, especially in the Croatian scientific and clinical community where there is a lack of studies regarding this topic. For this purpose, narrative literary review methodology was used and relevant studies, articles, books and other texts available at four scientific databases (PsychInfo, EBSCO, PubMed, CINAHL) and other sources like Google Scholar were searched primarily using the term “transdiagnostic” and using other relevant terms when focusing of research was needed. Available literature revealed transdiagnostic approach to mental health problems as a rapidly developing area in three relevant domains of scientific knowledge and technology: in classification and nosology of psychopathology, basic biopsychosocial research, and in clinical practice and the development of preventive interventions. In the classification and nosology domain, the most prominent alternative classification system identified was the Hierarchical Taxonomy of psychopathology (HiTOP). Instead of categorical it uses dimensional approach that describes psychopathology in quantitative terms with continuity of difficulties intensity from normal functioning to psychopathology. These dimensions are meant to be transdiagnostic. Syndromes are constructed from empirical covariation of symptoms and then grouped in spectra according to variations among them. As the name suggests, the model is hierarchical: at the lowest level it contains a group of homogenous components comprising a few hundreds of signs and symptoms and maladaptive traits. Moving upward these components are grouped in increasingly broader dimensions like syndromes, subfactors, spectra and superspectra and finally converge to the general factor of psychopathology. Till now, the model includes six spectra: internalizing, externalizing, thought disorder, disinhibited externalizing, antagonistic externalizing, detachment and somatising. Taxonomy is open to constant revisions as new evidence arrives. One of alternative ways for conceptualizing mental disorders that research revealed was a network approach to psychopathology. It conceptualizes psychopathology through the lens of complex, biopsychosocial, dynamic systems and hypothesizes that mental health problems are emergent properties of complex systems of symptoms that are in constant interactions. Besides conceptualization, the approach is aimed at developing new methodologies appropriate for identification of symptom networks and techniques for network analysis that can provide relevant information for clinical uses and creation of simulation models of disorders. One of the advantages of this methodology is the use of formal theories which describe relations among phenomena in mathematical terms. In the area of basic research and studying of basic biopsychosocial processes and mechanisms involved in aetiology and maintenance of mental health problems, search of the literature revealed several processes and mechanisms identified and proposed as transdiagnostic. Classification according to the range and nature of mental health problems they are connected to was proposed. One of the first transdiagnostic theories that involved multiple transdiagnostic processes responsible for the whole class of eating psychopathology is a Fairburn’s theory of eating disorders that involves primarily dysfunctional cognitive scheme for self-evaluation alongside mechanisms of clinical perfectionism, core low self-esteem, mood intolerance and interpersonal relations. Theory gave rise to the first transdiagnostic treatment, namely Cognitive behavioural therapy for eating disorders. Another example is Barlow’s and associates research on processes common to anxiety and depression. Temperamental trait of neuroticism was hypothesized to be connected to a whole range of anxiety and mood disorders and that core mechanism in emotional disorders involves interaction of negative or unpleasant emotional experiences and attempts of avoiding them. This led to a development of transdiagnostic interventions for adults with emotional disorders and followed in adaptation of interventions for the populations of children and adolescents. Some other proposed processes and mechanisms included self-focused attention, self-regulatory executive dysfunction, repetitive negative thinking, rumination and experiential avoidance, mentalization and models of interacting cognitive subsystems and schematic, propositional, analogical, and associative representation systems model. One of the most prominent projects in the field, from the beginning of the century, is the Harvey’s and associates research on cognitive-behavioural transdiagnostic processes that identified attentional, memory, reasoning, thought and behavioural processes connected to a wide range of psychopathology. Theoretical attempts that reach beyond explaining more narrow and specific relationships among phenomena included Perceptual control theory that suggests conceptualization of human suffering in terms of conflict between hierarchically organized control systems following ideas from cybernetics. One of the projects in the field of basic research was the Research Domain Criteria project started by the United States National mental health institute. Project is aimed at supporting new ways of classification and understanding of mental health disorders through developing a framework for research of pathophysiology primarily in the field of genomics and neuroscience, including dimensions of observable behaviour. Hierarchical scheme is proposed with five domains of functioning that had robust empirical support: negative valence systems, positive valence systems, cognitive systems, social processes, arousal and regulatory systems and sensorimotor systems. Every dimension is defined and contains the list of elements that can serve as measures of that dimension, on every of few units of analysis: genes, molecules, cells, circuits, physiology, behaviour, self-report and paradigms. The hierarchical taxonomy of the psychopathology system is open to updates as new evidence arrives. In the clinical domain some existing, long standing therapeutic approaches such as humanistic, cognitive or gestalt approaches to therapy are being recognized as transdiagnostic so as more recent approaches, for instance, Acceptance and Commitment Therapy (Hayes, Strosahl i Wilson, 1999). New interventions that target mechanisms involved in aetiology and maintenance of multiple disorders are also being developed, such as already mentioned Cognitive-behavior therapy for eating disorders (Fairburn, Cooper i Shafran, 2003) and Unified Protocol for Transdiagnostic Treatment of Emotional disorders (Barlow i sur. 2011a, b), that was also adapted for children and adolescents and modular interventions such as Modular Approach to Therapy for Children with Anxiety, Trauma and Conduct Problems (Chorpita i Weisz, 2009) or a Common elements treatment approach for adult mental health problems in low-and middle-income countries (Murray i sur., 2014). One of the most recent is the principlebased treatment protocol for a broad array of youth problems ‘FIRST’ (Weisz i Bearman, 2020). Preventive interventions domain is, in some ways, an independently developing field that is starting to recognize benefits of transdiagnostic paradigm and where transdiagnostic research regarding risk and protective factors alongside transdiagnostic preventive interventions is beginning to emerge. Thus far transdiagnostic research and practice has led to alternative classification systems such as hierarchical classification of psychopathology, alternative conceptualizations of psychopathology, numerous proposed biopsychosocial mechanisms underlying the development and maintenance of mental health problems, as well as interventions targeting comorbid conditions and a wide range of psychopathology. The field has a potential for fostering development of new paradigms in the field of mental health and for true integration of biopsychosocial domains of human functioning. Currently it is in need of developing theories that transcend diagnostic frameworks and suitable methodology that would integrate current knowledge and guide further research, and the development of practical applications.