Sometimes inline / text elements can appear vertically centered, just because there is equal padding above and below them.

.link {
  padding-top: 30px;
  padding-bottom: 30px;
}

See the Pen Vertically Centering Text by Chris Coyier (@chriscoyier) on CodePen.

If padding isn't an option for some reason, and you're trying to center some text that you know will not wrap, there is a trick were making the line-height equal to the height will center the text.

.center-text-trick {
  height: 100px;
  line-height: 100px;
  white-space: nowrap;
}

See the Pen Centering a line with line-height by Chris Coyier (@chriscoyier) on CodePen.

Equal padding on top and bottom can give the centered effect for multiple lines of text too, but if that isn't going to work, perhaps the element the text is in can be a table cell, either literally or made to behave like one with CSS. The vertical-align property handles this, in this case, unlike what it normally does which is handle the alignment of elements aligned on a row. (More on that.)

See the Pen Centering text (kinda) with Padding by Chris Coyier (@chriscoyier) on CodePen.

If something table-like is out, perhaps you could use flexbox? A single flex-child can be made to center in a flex-parent pretty easily.

.flex-center-vertically {
display: flex;
justify-content: center;
flex-direction: column;
height: 400px;
}

See the Pen Vertical Center Multi Lines of Text with Flexbox by Chris Coyier (@chriscoyier) on CodePen.

Remember that it's only really relevant if the parent container has a fixed height (px, %, etc), which is why the container here has a height.

If both of these techniques are out, you could employ the "ghost element" technique, in which a full-height pseudo element is placed inside the container and the text is vertically aligned with that.

.ghost-center {
  position: relative;
}
.ghost-center::before {
  content: " ";
  display: inline-block;
  height: 100%;
  width: 1%;
  vertical-align: middle;
}
.ghost-center p {
  display: inline-block;
  vertical-align: middle;
}

See the Pen Ghost Centering Multi Line Text by Chris Coyier (@chriscoyier) on CodePen.

It's fairly common to not know the height in web page layout, for lots of reasons: if the width changes, text reflow can change the height. Variance in the styling of text can change the height. Variance in the amount of text can change the height. Elements with a fixed aspect ratio, like images, can change height when resized. Etc.

But if you do know the height, you can center vertically like:

.parent {
  position: relative;
}
.child {
  position: absolute;
  top: 50%;
  height: 100px;
  margin-top: -50px; /* account for padding and border if not using box-sizing: border-box; */
}

See the Pen Center Block with Fixed Height by Chris Coyier (@chriscoyier) on CodePen.

It's still possible to center it by nudging it up half of it's height after bumping it down halfway:

.parent {
  position: relative;
}
.child {
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
}

See the Pen Center Block with Fixed Height by Chris Coyier (@chriscoyier) on CodePen.

No big surprise, this is a lot easier in flexbox.

.parent {
  display: flex;
  flex-direction: column;
  justify-content: center;
}

See the Pen Center Block with Unknown Height with Flexbox by Chris Coyier (@chriscoyier) on CodePen.