TOOL DEVELOPMENT All potential information about the project can be given verbally (this will be over the phone in the first instance). For CYP (children and young people) that are under 16, parental consent will also be needed. After the phone call, if individuals are interested in taking part, age appropriate information sheets will be posted out to both the CYP and guardians/parents outlining the details of the study. The researcher will discuss any questions potential participants may have over the telephone, and both will have as long as they need to decide whether to take part. If they decide to take part the participants will be invited to take part if they meet study inclusion criteria. For clinicians, the researcher will come to staff meetings, outline the study and leave contact details/expression of interest forms so that those who are interested can get in contact to find out more. For young people and families, clinician's will briefly outline the study and interested participants will fill out an expression of interest form. After the researcher receives the contact details participants will be contacted by a researcher so that further information researcher will then arrange a time to meet with them for an interview or when they may be available for a focus group, and consent forms will be signed. Clinicians, parents/guardians and those aged 16 will also have the opportunity to have the interview conducted over the phone if they prefer (in which case consent forms will be posted). During interviews participants will speak about their previous experiences and beliefs about SDM as well as view and discuss the few existing SDM tools used with young people within CAMHS to use as a baseline for further development. After the interview participants will be debriefed, and be given the opportunity to validate themes that emerge from the data once analysed.

USING POWER UP IN CAMHS SERVICES Clinicians For clinicians, the researcher will be come to staff meetings, outline the study and leave contact details/expression of interest forms so that those who are interested can get in contact to find out more. After the researcher receives the clinicians contact details they will be contacted by a researcher so that further information about the project can be given verbally (this will be over the phone in the first instance). The researcher will then meet with those who are interested an outline the study to them (including randomisation). The researcher will discuss any questions potential participants may have over the telephone, and both will have as long as they need to decide whether to take part. Those who want to take part will be asked to sign a consent form, and will be allocated to use 'Power Up' or continue as TAU with their patients.

CYP/ Families Young people and their families will be invited to take part if they meet the study criteria. Clinicians will briefly outline the study and interested participants will fill out an expression of interest form. After the researcher receives the contact details patients and their families will be contacted by a researcher so that further information about the project can be given verbally (this will be over the phone in the first instance). For CYP's that are under 16, parental consent will also be needed. After the phone call, if individuals are interested in taking part, age appropriate information sheets will be posted out to both the CYP and Guardians/Parents outlining the details of the study. The researcher will discuss any questions potential participants may have over the telephone, and both will have as long as they need to decide whether to take part.

If they decide to take part the researcher will then arrange a time to meet with them at the CAMHS service and complete any baseline assessments (week 1) for clinical symptomology (e.g. Revised Child Anxiety and Depression Scale , Strengths and Difficulties Questionnaire) if these are not routinely collected by the CAMHS service. Randomisation will occur at clinician level and participants will be made aware of this in the information sheets. Throughout the course of treatment session by session measures at weeks 2, 4 and 8) will be completed. Participants and clinicians will also be asked if they consent to the researcher observing one appointment session when the tool is being used (this is optional and will last up to 30 mins, but will likely be week 2 or 3) to understand how the tool is being used in clinical settings.

At the end of treatment (approximately week 8) clinical symptomology, therapeutic alliance, level of shared decision making, and experience of care will be collected. Participants will be invited for an interview to hear their views and experiences about the 'Power Up tool' which like other interviews will take place at a time of their choosing and abide by SOPs (standard operating procedures). Participants will exit the trial and any further treatment will be discussed between them and their healthcare professional.