The number of teachers who have achieved chartered status has passed 1,000, the General Teaching Council for Scotland announced - eight years after the scheme's launch in 2002.
Chief executive Tony Finn said it was a "significant milestone" for a programme that was making "a real difference" to teaching and learning.
But Ken Cunningham, general secretary of School Leaders Scotland (SLS), said: "We've still got a way to go for it to make the impact that the profession would like."
Heads had hoped for considerably more than 1,000 chartered teachers (CTs) by now.
SLS wants more clarity about funding and the chartered teachers' role. Mr Cunningham also called for "more openness and transparency" about the selection process.
But he added that chartered teachers had become an integral part of schools. The programme had recovered from a slow start, when the quality of chartered teachers had been variable and there was scepticism that some were signing up primarily to boost their pensions.
The programme had already been a "significant success", said the Educational Institute of Scotland's general secretary, Ronnie Smith, and it was "essential" to keep encouraging more people to sign up.
"In this time of budget cutbacks and a lack of resources for continuing professional development, we will need to ensure that access to chartered teacher programmes is not cut back or curtailed," he said.
"The decision to follow the chartered teacher route is always one for the individual teacher, and should not be influenced by budget concerns at local authority level."
The "resistance" seen in the programme's early days had gone, said national CPD co-ordinator Margaret Alcorn. "I think it's beginning to come into its own," she added. "We're beginning to see examples of excellent practice emerging."
A group of 31 teachers were awarded CT status at a ceremony in Edinburgh this month, taking the total up to 1,010.
Henry Hepburn, email@example.com.